The giant spider you face at the beginning of the Canopy level has no doubt scared some players. The music that accompanies that part of the level doesn't help matters either.
The Zombie Chickens.
The way Foutch sneaks up on Rayman and slugs him so forcefully he loses one of his "permanent" powers: flying. Yes, he hits Rayman so hard that rayman forgets how to/is unable to how to fly! Damn.
Eig in the PS2 versions, a horrific monster that appears in the Marshes of Awakening, but only makes a split second appearance. Way later, in the Sanctuary of Rock and Lava, he appears out of the water next to a secluded log. If Rayman approaches him, he'll appear to eat him and Rayman dies.
The giant mechanical robotic dinosaur that guards the Iron Mountains is surprisingly fast for its size, something most players find out as they speed away from it on their walking shells, only to get trampled and killed instantly.
The music in the Tomb of the Ancients (especially the Psycho Strings bits) is creepy on both the N64 and Dreamcast versions, but the Dreamcast version is just... it's just... *shutter*