As you may have noticed, I've been using a very loose definition of the term "pet". Basically, I think any interactive animate being the player owns or controls can be considered a pet. That vague definition includes humans, which wouldn't normally be considered pets, but in games like the Sims they certainly function like pets. Pets function so differently across the spectrum of games that any more narrow definition would exclude some pet games. But, if you would prefer to define pets a bit more narrowly, that's fine, because this section is all about narrowing down which type(s) of pet you want in your game!
Some common kinds of pets:
Avatars As Pets - The most minimal element that might justify calling something a pet game is when the playable character looks like an animal or monster. This would include all games where the player is a puppy, horse, dragon, etc. The playable character does not act like a pet
Vanity Pets and Transformations - Vanity pets are the simplest kind of pet because they have no gameplay functions aside from character customization. In other words they are just there to look pretty following the player's avatar around or living on the player's property, thus the term 'vanity'. In some cases they may have minimal interactivity, such as needing to be fed or occasionally saying randomized dialogue. They typically wander around a bit, though anchored to the player's avatar or to a placement point on the player's property. Or they may wander around inside a tank or pasture. Transformations are when a normally human avatar is shapeshifted to look like a pet, but this does not affect the way the avatar functions (emoticons or chatting might be affected, but those are also 'vanity' elements). Sometimes vanity transformations are the lowest level of something upgradable into a functional traveling form or combat form.
Mascots - This is any kind of companion character that gives the player feedback on their actions. Clippy from Microsoft Office, Navi from the Zelda series, and Issun from the Okami series are some examples of this 'helpy' type of pet. By 'companion character' I mean an NPC which follows the player around, is a part of the game's GUI or always available via menu, or pops up regularly to deliver tutorials, tips, and/or narration. A mascot may function as a tool, such as a compass, radar, grappling hook, digger, or backpack-carrier. Perhaps the most developed case of a mascot-type pet ever is the blob from game A Boy and His Blob - this was originally an NES game but a new version is currently available as a wii game. This is a puzzle game where the pet blob functions as a swiss army knife of tools used to help the avatar travel through the level.
Mounts and Traveling Forms - This is any pet used as a vehicle in a way which is more than just looks - the creature must actually increase the avatar's speed or be capable of movements the avatar is not, such as jumping, swimming, or flying. In some games (many racing games for example) the avatar is not capable of moving at all without their vehicle. A mount is a pet the player sits on or is otherwise carried by; a traveling form is a shapeshift which transforms the player into the pet. An interesting classic example of pets as vehicles is the NES game Little Nemo the Dream Master; this game is a platformer with many puzzles that must be navigated by using different pets as vehicles, each of which have different abilities. Mascots are sometimes usable as mounts or transformations
Infrastructure Pets - These are pets which produce or process resources. Cows produce milk, sheep produce wool, hens produce eggs, and various animals produce manure, feathers, pearls, or fantasy resources like gems and mana orbs. Beavers might process trees into logs or boards, fire-breathing dragons might process ore into ingots or wood into charcoal, hens might incubate dinosaur eggs to hatch them into dinosaurs; the possibilities are infinite. Infrastructure pets can even include alien or fantasy creatures which function as buildings or furniture. Typically infrastructure pets are found in sim and RTS games, though they would work in any game where the player has a "home base" which they upgrade between missions.
Combat Pets and Combat Forms - This is any pet which which increases or alters the player's abilities in combat. The creature may fight alongside the avatar, fight under the direction of a non-fighting avatar, be equipped onto the avatar as a weapon or armor, or be a transformation of the avatar. This includes all Pokemon, all Monster Rancher monsters, all creature units in tactical combat games, all pets of the Hunter and Warlock Classes in WoW, all non-vehicle transformations of the Druid Class in WoW, and vehicle pets which also have combat abilities. The most common kind of pet in games which have combat.
Capturable Pets - The flip side of combat pets, these are creatures that fight against you. (They often turn into combat pets after you capture them, though they can also turn into infrastructure pets or become available as shapeshifts.) There are two main dynamics of capturing pets - either you fight them to weaken them then must capture them before they are quite dead or run away, or you must NOT fight them, instead distracting them with bait or getting them to chase you into a trap or enduring their attacks while you wait for your capture spell to be cast or a tameness meter to fill. It's also possible to have capturable pets in a game with no combat - they might be randomly encountered while exploring and captured by paying money, using a consumable item such as a net or collar, or playing a minigame where a sufficiently high score is needed to capture the pet. Capturable pets occur mainly in RPGs.
Collectible Pets - Games with collectible pets are also called "Gotta Catch 'Em All" games. They typically give the player quests or achievements for accumulating numbers or sets of pets, and keep track of the first time the player collects a pet of each type in a book or list. Pets can be obtained either by capturing or by breeding, and this kind of pet occurs mainly in RPG and sim games. Pocket Frogs and Fish Tycoon are two examples of collection by breeding instead of capturing. (Plant Tycoon is a similar game with somewhat different/improved features, if you consider plants to be pets). But unfortunately, neither of these games are good examples of a collection quest/achievement system. Monster Rancher is slightly better - it keeps track of your discovered pets in a book and notified you when you have found a new one. Still no quests or rewards for breeding though.
Breedable Pets - Breeding pets can be a large portion of the gameplay in some pet games. In some games the player may be breeding pets of specific appearances to sell to customers, either by filling an NPC's order or in a pet shop where each type of pet has a set price. In other games the player may be breeding for stats, related to combat or competition. The game does not actually have to contain competitions; Celebrity Pedigree is a game where pet quality is measured in stars, and the overall goal is to have a certain number of 5-star (maximum quality) pets. The game's theme implies that the pets are competing as celebrities in the entertainment industry, but they presumably are entered into this competition by their new owners after you sell them.
Craftable Pets - Aside from breeding or capturing, pets can also be produced by crafting. For example an artificer might build clockwork and golem pets, or a mage might gather spell ingredients to summon a mythological pet with a magic potion or ritual, or a sci-fi toy factory might assemble pets on an assembly line. Less literally, an NPC might give a pet to the player in exchange for a batch of ingredients or crafted items. Craftable pets would be especially suited to time-management games, where the player would run their avatar around at top speed to efficiently gather the crafting ingredients to fulfill pet recipes. But craftable pets could occur in a sim, RPG, etc. Craftable pets can be combined with capturable or breedable pets if the pet captured or bred is a basic type which can be developed into a more advanced type.
Consumable Pets - These are creatures which, once captured, bred, or crafted, can be used once or a set number of times to give some particular stat bonus, work, or other assistance to the player. The fairies in the Zelda series which can be captured in bottles and stored until the player needs to be healed are consumable pets. In a game where the character is a summoner, summoning a pet into combat might consume it or wear it down.
More About Genetic Systems:
Any type of pet which is breedable needs to have some type of genetic system. This may, but does not have to, bear any resemblance to real genetics (which IMO don't make for great gameplay). Sometimes it bears a resemblance to the discredited concept of Lamarckian inheritance instead - this is a system where offspring inherit a stat bonus based on their parents' training and accomplishments, instead of their parents' genetics. This is nice, gameplay-wise, because it means that successive generations of the same type of pet can reach slightly higher ranks of competition (or whatever) before they must be retired (assuming the game has pet aging). It's like a "new game plus" for individual pets. Many pet breeding systems do not have pet gender, because it simplifies things for the player if any pet can be bred to any other pet (sometimes including itself) and if there are no sex-linked genes or non-genetic appearance variation by gender.
Genetic systems can be about appearance only, stats only, or both. The stats part is pretty simple - a type of pet may have a basic set of stats with possible bonuses from inheritance, or an individual pet's stats may be entirely determined by its parents' stats, with minor randomization. Often the genetic process is slightly biased in favor of offspring getting good stats. So, for example, if HP is one of the pet's variable stats, the genetic algorithm could average the HP stat of the two parents, the randomly choose a value in the range from (average - 20%) to (average + 35%). Or if that type of pet had a base HP stat, you would subtract that base from the average, randomize as above, then add that back to the base. That's just one really basic example - there are all sorts of ways to design the math behind inheritance in your game.
Appearance inheritance systems can be done in three ways:
The first way is that there are set pet shapes, but each shape can come in different colors. So a red pet of type A and a blue pet of type B could be bred to result in a blue pet of type A, among other combinations. In some cases purple pets might also result from breeding a red pet with a blue pet. In a 2D bitmap/raster system the pet shape would consist of a lineart layer (unless it's a lineless art style), coloring layers, and shading layers (each layer would also have a layer mask, with the possible exception of the lineart layer. The pet's color would be changed by bucket-filling or mathematically rotating the color layers. The final sprite is flattened and saved as a PNG file with transparency. In a 2D vector system there are no masks and each color-area is its own object on its own layer. But the basic principle is the same, an image built up from color fills and areas of shadow and highlight. The color can be changed by bucket-filling but it's more efficient to do it mathematically, with the search-and-replace function. For example you can tell the vector graphics program, "Replace all areas of color #FF0033 with color #AA00FF." The final image can be an SVG file or can be flattened and exported as a PNG. In a 3D system the pet shape is a model and the color is a texture or procedural color applied to it. For the final version the model and textures can be stored separately in any of several file types and combined by the game engine, or the model and texture(s) can be bound together, or the textured model can be rendered to a 2D sprite.
A second way of doing appearance inheritance is having body parts with set colors which can be mixed and matched. The 2D version of this system, whether bitmap/raster or vector, is called a paper doll, and is the same kind of thing as a 2D avatar clothing system. Each body part is an image file and the game assembles them in a predetermined layering order. (For clothing the game might let the player choose the layering order, but this is unlikely to be useful for pets.) The 3D version of this system divides the model into smaller models, each of which has its own texture.
The third way combines the first two - the pets have both mix-n-match body parts and a genetically determined color scheme. Some systems have one or two colors (main and accent) for each body part, but the results may look nicer if the color genetics determine a scheme of 2-4 colors for the whole pet and this scheme is applied to the body part shapes selected. Really sophisticated 3D systems like that in Spore can have the body part models be adjustable and the connectors between them generated procedurally, instead of both the body parts and connectors being of predetermined size and position.
Appearance inheritance is a bit less math-friendly; in some cases you might need something more like a spreadsheet or some rules expressed through if/else programming loops (or whatever your programming language of choice calls them). I mean, you can convert colors into hex values and then do math on those, but that can be ugly, in more ways than one. And converting body parts into numbers is also dubious, unless you are talking about numbers for dominance and recessiveness, which only apply to polyploid systems (systems where each creature has two or more sets of chromosomes). Many pet systems are monoploid for simplicity, because in a monoploid system genotype is always the same as phenotype. If you did not understand that sentence you would have to go study genetics before you would be able to design a realistic genetic system. Having a set color palette of somewhere between 10 and 40 colors that pets can come in, and/or a list of shapes each body part can come in, is my personal favorite approach.
Example color palette spreadsheet: [Ugh formatting fail, sorry, I'll have to figure out how fix this later...]
[table]
[tr]
[td]White[/td]
[td]L. Brown[/td]
[td]L. Orange[/td]
[td]L. Red[/td]
[td]L. Purple[/td]
[td]L. Blue[/td]
[td]L. Green[/td]
[td]L. Yellow[/td]
[/tr]
[tr]
[td]Gray[/td]
[td]Brown[/td]
[td]Orange[/td]
[td]Red[/td]
[td]Purple[/td]
[td]Blue[/td]
[td]Green[/td]
[td]Yellow[/td]
[/tr]
[tr]
[td]Black[/td]
[td]Dk. Brown[/td]
[td]Dk. Orange[/td]
[td]Dk. Red[/td]
[td]Dk. Purple[/td]
[td]Dk. Blue[/td]
[td]Dk. Green[/td]
[td]Dk. Yellow[/td]
[/tr]
[/table]
The top, middle, or bottom row is determined by a brightness gene which can be A, B, or C. Assuming the system does not have dominance and recessiveness, the inheritance algorithm could look like this:
A + B = 50% A, 50% B
B + C = 50% B, 50% C
A + C = 25% A, 50% B, 25% C
The color inheritance rules would be a bit more complex:
Orange + Red = 50% Orange, 50% Red
OR 25% Orange, 50% Brown, 25% Red
Orange + Purple = 25% Orange, 50% Red, 25% Purple
OR 25% Orange, 25% Red, 25% Brown, 25% Purple
OR 100% Brown
Orange + Blue = 30% Orange, 30% Blue, 10% each other color excluding brown and gray
OR 25% Orange, 50% Gray, 25% Blue
OR 100% Gray
Brown + Brown = 40% Brown, 10% Each other color excluding gray
OR 100% Brown
There are many, many other ways this could be done. For an utterly different but still monoploid genetic system you could google a walkthrough of Plant Tycoon, and there are other walkthroughs and wikis which explain the breeding systems in various games. I just wanted to give an example of what genetic algorithms might look like. Now lets finish up with your features' list entry for this section:
- [For the primary type of pet in your game, which category(ies) do they belong to?]
[indent=1]- [Describe how the pet's functions within game fit primary category.]
[indent=1]- [Describe how the pet's functions within game fit each additional category.]
- [For each additional type of pet, which category(ies) do they belong to?]
[indent=1]- [Describe how the pet's functions within game fit primary category.]
[indent=1]- [Describe how the pet's functions within game fit each additional category.]
- Pets inherit [appearance, stats, both, or skip this section if there is no inheritance]
[indent=1]- [If there is stat inheritance, how does it work?]
[indent=1]- [If there is appearance inheritance, how does it work?]
[indent=2]- [2D bitmap/raster, vector, or 3D]
[indent=2]- [set forms or mix-and-match body parts]
[indent=2]- [set colors per form or body part, standard palette of colors, numerical]
