Aside from the Horta, and a couple glowing space things, basically every alien in The Original Series was a quickly painted extra.

How could you forget the [Alfa 177 canine](https://memory-alpha.fandom.com/wiki/Unnamed_non-humanoids_(23rd_century\)#Alfa_177_canine)? 😁 (and, on a more serious note, the M113 salt creature which was my introduction to Star Trek and which looked pretty awesome)