Well the Zerg''s original idea was great.
Zerg were little parasitic creatures, enhanced by the Xel Naga (some really intelligent god-like species which the Zerg killed in the end) to form a collective.
The Zerg crept into a host''s spine and controlled their mind in such a way.
Eventually, they became very hostile, even to each other and started to modify their host creatures. That''s where it became a more ''standard'' alien species and they just started to destroy everything in their way.
After infesting everything in their planet, a species which could travel through space without vehicles came along and the Overmind (the ''collective'' master) sent a telepathic signal to them and so the Zerg got control over them and added them to their genes.
Oh by the way, before all that the Zerg mutated to assimilate the hosts DNA into their own and somehow into the collective.
This became the Zerg Larvae which can (in the game) be selected to mutate into any creature which has ever been infested/assimilated.

All I''m saying: Zerg weren''t bad but in the game itself they''re pretty default aliens.

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Allow me to clear my head for once...
Stop polluting the air!

Most films/TV shows/books that deal with alien lifeforms are more concerned with the biological differences among them that the creatures habits, personality, even language are only given generic descriptions...

Example: Klingons...everybody knows what they are...they are a tough ''warlike'' race that speaks the Klingon language...but think about then and the way they are used in stories, etc...they are basicly stereotypes and little more...

For a planet as small as the Earth is...we humans have a lot of variety...different languages, cultures, habits, intrests...even other animal lifeforms on our planet show a huge amount of diversity (even biological differences among members of the same species).

So to create truly realistic aliens you are going to have to address this diversity...even embrace it

Hmm. A more interactive alien. Throck

Habitat

Throck lives in air and breathes oxygen and nitrogen.

Physical Characteristics

Throck is a Gestalt lifeform, formed from many indepedent, but identical, parts. In this respect it most closely mirrors the Dictyostelida (celluar slime molds) of Terran zoology.

Like slime molds, Throck exists in two primary forms: single-celled, and multi-celled.

Single-celled

In this form, Throck may be present as a heavy mist, or a clear slimy coating on objects in its habitat. Single-celled Throck has no cognitive ability.

Multi-celled

In this form, Throck may have the appearance of a clear jelly. Over time, it is able to add pigment to any of its cells. When multi-celled, Throck has considerable cognitive ability, depending upon the size of the assemblage - about two kilos of Throck is as intelligent as a human. At forty kilos, the intelligence levels off to a maximum.

Throck cells can move at a maximum rate of about 10m/s. They are also able to solidify, forming bone-like structures. The bone can be formed quickly, but takes time to destroy. The bone-forming reaction is exothermic - the Throck gains energy from solidifying, so a Throck that is starving is likely to become completely solid as it dies.

Food

Single-celled Throck consumes bacteria. Naturally, these are native to the Throck''s planet of origin, so bacteria from any old planet won''t do.

Multi-celled Throck can eat just about anything that is digestable. However, it can do so only slowly. It is most efficient at digesting nutrients dissolved in water, and of course it can eat bacteria just like it''s single-celled form.

Psychology

Throck isn''t used to recognising people by physical appearance, instead depending upon smell and personality. If a person suddenly starts behaving differently, a Throck may believe that a different person has replaced him, even if it happens in the middle of a conversation.

For Throck, death is not usually an issue: Throcks are very hard to kill, and even if only a few cells remain after they are attacked, they can grow back into a sentient Throck, although their memories and learned cognitive skills will most likely be impaired.

Social Structure

The distinction between one Throck and another is blurry. Whenever two Throck-masses are in contact, there will be two seperate intelligent entities, but the exact boundaries between the two will be forever shifting.

Generally, therefore, Throcks do not tend to have a social structure at the small scale. Instead, social structures will be formed for geographically distant Throck colonies. Often, these will be on a peer-to-peer basis, with no colony being more important than another.

Interaction Tips

Threatening an individual Throck is like threatening an individual hair: although your victim may not want you to pull his hair out, he knows he''ll grow another one. The only way to threaten a Throck is to threaten to destroy the entire Colony.

Throck may be confused by local-scale chains of command. If a captain and a commander meet with a Throck, it may not understand how the captain could be more important than the commander. It may not even understand how the captain could be a different being from the commander.

Throck may kill somebody that is in their way without the malice you might associate with such an action. From a Throck''s perspective, killing a single body is like pulling a single leaf from a tree.

Throck may expect you to know everything that has happened to people you have had recent contact with. In particular, if a collegue of yours has talked with a Throck, which you then meet, it will not think there is anything wrong with continuing the conversation from where they left off.

All your bases belong to us (I know. It''s irony.)