X-rays is a classic as well as ET. I do use bugs too, esp for sectoids or Chryssalids. Chryssies comes out, muties too sometimes.
Discs, & Frisbees obvious for CyberDisks. Beasties for Reepers. Green Meanies sometimes for Mutons.
(Tftd: Gillies: Gillmen, "Swamp Thing": Deep One, Shellbacks: Lobstermen, Lizardmen/iguanas: Tasoths, and of course Flying Brains for Tentaculats.)
More generally, enemy, adversary, foe, hostiles, etc.
Doing an LP made me want to develop a decent lexicon, to avoid repeating myself... though, now that I think about it... it was more doing writing that made me conciously think about it. Combat Diaries for Marbozir's Long War LP, and then the Soldiers of Earth Roleplay made me really think about not using the same term multiple times in a row.
Cheers, Ivan