There is an interesting documentary on youtube about development of UFO:EU:
https://www.youtube.com/watch?v=LP7VjbuNEzgNow, if you stop it at 13:50, you'll see some aliens projects that were considered for the game. Maybe some of them will inspire you for making new species
(I see a Holodrone there, so probably someone already found this. In that case please delete this thread)