I'm working on quest and level design for the first post-tutorial area of an RPG. The game has an overall requirement that the monsters must all be small at the beginning and most will get larger as the game progresses. Also the first area monsters should seem like appropriately mild dangers to send a student or teenager to deal with as a training exercise. So, of course many RPGs have done the same sort of thing. Slimes, rats, bats, beetles, spiders, carnivorous plants, and mushrooms all seem like very common choices for this type of area and purpose. They would also all do fine for the area's secondary purpose of introducing players to the monster capturing and breeding system, especially the beetles, rats, and slimes. The question is are they too common? Would you be disappointed or annoyed to enter a fantasy world and find yourself asked to fight one of these kinds of monsters and capture them as starter pets? Would something more exotic like, oh, knee-high mini tyrannosaurs which the game states to be a mildly annoying local pest, be a better choice?

For my part, I don't mind more common monsters provided that there are plentiful alternatives as well. I suppose it would also depend on how long I'd be stuck with that particular monster, which would also affect how much I see them around with other people. I think that I would probably prefer a more original set, but to do that you might be forced to make stylistic choices you would not need to with more common types. These could be a bit of a turn off for some players.

anything but rats. Why out of anything does it need to be rats, goblins or kobolds.

I dunno if this was a rhetorical question or an actual question. But rats/mice are a common choice along with spiders and such because they are something people may have actually killed or wished they could kill in their homes or other local environment. Also rats have an association of carrying sickness, while spiders have an association of being poisonous and a bit vampiric.

I sort of like having to start out fighting little rats that are easy to kill. If you start by teaching someone how to fight something like a one eyed purple flying unicorn, it gives a slight impression of unrealism, and almost specialization. Something unique will have unique strengths and weaknesses the player will have to pick up on later, but the first few missions whose purpose is to teach someone how to use the attack button should not also expect someone to remember that "to kill a purple flying cyclops unicorn, play the note on your flute corresponding to the blue button, then etc."

Also, as a game progresses, enemies typically get more outrageous and flashy, forcing the player to continue just to see "what kind of monstrosity could the designer come up with that's better than this monster?" Starting off against exotic monsters implies a lesser degree of gradient between exoticness of a monster, which could lead to boredom.

One thing to watch for, however, is excessive "teaching", or slateness. One game made me kill hundreds of armadillos using a bow and arrow (as a tutorial on attacking) before progressing to the next point in the story. Needless to say, 5 or 10 would suffice for teaching how to right click something. Don't force the player to focus on only one "stale enemy". Give them a choice between goblins, rats, tumble-weeds, orange slimes, etc as possible training subjects. If they can make a pet out of a defeated monster, rather than only having "get a pet rat" as a tutorial, different people will prefer a slime as a first pet, or a goblin servant.

Khaiy says it well

[color="#1C2837"]For my part, I don't mind more common monsters provided that there are plentiful alternatives as well.
[/quote]

The good thing about using the same common choices that pop up in most other games is that they will feel familiar with players and you can therefore rely on existing knowledge rather than having to explain everything. This should leave you free to explain what players need to know about your game rather than wasting time explaining that in this setting a knee-high-tyrannosaurus is a simple pest; this will probably be particularly applicable to you if you're introducing the player to one or more mechanics they might not find familiar from other RPGs.

One thing to watch for, however, is excessive "teaching", or slateness. One game made me kill hundreds of armadillos using a bow and arrow (as a tutorial on attacking) before progressing to the next point in the story. Needless to say, 5 or 10 would suffice for teaching how to right click something. Don't force the player to focus on only one "stale enemy". Give them a choice between goblins, rats, tumble-weeds, orange slimes, etc as possible training subjects. If they can make a pet out of a defeated monster, rather than only having "get a pet rat" as a tutorial, different people will prefer a slime as a first pet, or a goblin servant.

To avoid boredom, I don't see changing the monster model used as being essential, changing the AI it uses and if possible the combat animations are much more effective at making it feel like something different.

In this particular game the player is intended to capture every monster if he or she decides to participate in monster-capturing gameplay at all. The initial area will have something like two or three colors of monster species A and two or three colors of monster species B. I don't think it's really a problem to tell people to capture a "brown A" first since as soon as they accomplish that they will be expected to also capture the other options (if they are following the optional monster capturing track through the game). They will further be expected to try breeding all possible combinations of these 4-6 types of monster before completing the initial area, which will reveal two or three breedable-only versions of these monsters, including one completely new hybrid type.