Unfortunately I think you might have to do a short write up on each one. If someone wants to use your planet in an RP and then they get attacked by a creature they could make up all sorts of things like it having poisonous fangs when it's not supposed to or a tongue that can extend ten meters...
How short do you think I could get away with though? :P
I mean, there is no way I'm making twenty half-sized species write-ups, but if they are allowed to be like five sentences or so, then I'd probably do it.