After rewatching the original Star Trek series once again, I noticed that really few non-humanoid creatures appeared in the show. The Horta was one of the few we got to see and even though the creature design is rather simple I was inspired to paint one after seeing the episode.
Luckily WizKids produced a Horta for their Star Trek HeroClix: Away Team series. Of course it was cast in abysmal soft plastic and it was prepainted, but as the Horta does not really sport too many details this was not a big problem.