I read somewhere either Mike Sussman or Manny Coto wanted to do the tiny gold people from "Journey to Babel".
Would've been interesting to see if there was a way they could do a Medusan.
I suppose continuity would make seeing a Horta almost impossible, unless there was another "mirror universe" exception. (Pity, since a CG Horta would be interesting to see).
How about the people of Tiburon?