Your aliens are limited only by your imagination. They can come from worlds wholly unlike Earth. Their native habitat can be a primordial ooze, a methane slush, the surface of a sun, or the vacuum-parched boulders of a tumbling asteroid.

They can be inspired by mythology, or by your worst nightmares. They can have very different biologies from our own, or demonstrate ways of thinking and feeling that to us are bizarre or incomprehensible. If you don't want them to be carbon-based life forms, they don't have to be. They don't have to have physical bodies at all. You really have the most amazing license to be wildly imaginative.

But remember what I said earlier about freedom always implying limits? Well, that applies here, too. You have freedom, yes—but if you want your aliens to be believable, you must give thoughtful consideration to who or what they really are, and to where they live. If they have bodies (most do), their bodies should reflect the environment they live in. Critters that spend their lives floating among hydrogen clouds are not going to have the same body form or internal physiology as their brethren who live at the bottom of the sea. And neither one would last long in the airless void.

I'm not trying to take away your fun. On the contrary, much of what is fun about alien-designing is the challenge of thinking up strange environments for creatures to live in, and then figuring out how to make them believable.

Another thing—the physical form of your alien is only half the problem. You need to ask: How do they think? What do they feel? What sort of society do they live in, if they have one? Your aliens' hearts and minds should reflect the cultures they live in.