Ah, the Fiend Folio. That was a 1981 fever dream including a bunch of now classic D&D monsters. Without the Gith races, Baldur's Gate 3 wouldn't be the same. It also introduced the stereotypical bird-men in several settings, the Aarakocra. It included the Kuo-Toa, which became famous with the D series of adventure modules (specifically D2, Shrine of the Kuo-Toa) that also solidly introduced the Drow as big bads. D&D wouldn't be the same without Ettercaps, Iron Cobras, Mephits, or Slaads.
It was also full of the kind of D&D weirdness that was tailor-made to allow a Dungeon Master to mess with players. The Adherer would stick to your weapons when you struck them. The Carbuncle was an armadillo with a precious gem on its head, but you had to non-violently coax it to give it up (which was a problem for many players). The Disenchanter would ruin your precious magic items. The silly-named Flail Snail had a shell that could reflect spells back on the caster (and the drawing looked exactly like you think it would). The Gorbel was a rubber-skinned limpet that would explode when pierced. Needlemen are humanoid plants that "appear to hate elves and will attack on sight." A lot of it didn't make a ton of sense but it was really fun.
