If someone commissions you to draw your character, sure, it's fine - USUALLY. It's when you start making money on characters based one something that's been trademarked you start to get into a legal grey area; the Oddworld species/characters are trademarked/copyrighted (I forget which - don't think you can copyright an invented species, but you can certainly trademark them) to OWI, and since a company wouldn't appreciate non-company people making money off them... It'd be like selling things under the Sainsbury's brand name, fr'instance, while the company can't copyright the name they can trademark it and you'd be infringing that registered trademark.
Any sci-fi creature or character is the intellectual property of the person that created it, I think it goes. And while it's not strictly illegal to create fanfiction/characters, it'd probably be illegal to make money off it. Even if it isn't, OWI could sue (doubtful but stranger things have happened), and I don't fancy having to fight it out in court.
As a sidenote, I have more art:
Nyen - the Nyen are a race of aliens from my fiction universe; not a very nice race of people but then someone's got to do the dirty-work... I have very much fallen out of love with this picture as the colouring is so blooming shoddy (I got fed up of it and it went all mooshed towards the end)