The Black Dossier is a bestiary for FIST.
Its PDF is 22 pages, with a dense but atmospheric layout and a lot of cool ideas and some solid development of the lore surrounding CYCLOPS and FIST.
The bestiary's focus is primarily on extraterrestrials, but there's some other types of threats thrown in as well---soviet undead, cybertech apes, etc.
And even within the single theme of 'aliens', there's a lot of variety. The foes here are factions, not one-off boogeymen, and this creates a lot of potential depth for missions involving them.
None of the stat blocks are super complex, so if you prefer having lots of special abilities you'll have to add in your own, but there's plenty of hints on what to modify in the foes' text.
Overall, if you want some more creatures to throw at your players, or you want to lean your game of FIST in an X-Files-y direction, I'd recommend giving Black Dossier a try.