Oh, the bullywugs and kuo-toa. On the surface, the frog-men and the fish-men seem so similar. Underneath, they are oh-so-different. Bullywugs are just brutish frog-dudes (though in Greyhawk, they were transformed into allies of sort of KKK-ish cultists, but that's another story.) They are weird, creepy and animal-like. Ho-hum.
The Kuo-toa are more my cup of tea. Sure, they didn't originate in the Fiend Folio, but I discovered them in the Fiend Folio before I discovered them in Module D2. The Kuo-toa are inhuman fiends, with strange social specializations in their society, alien motivations, and deep Lovecraftian weirdness in their look and feel. They use strange weapons, they are unpredictable. They're creepy.
They'd be great to find in an underground lake somewhere in Athanor.