I think we all had a certain fear of these guys who ruined our peaceful and happy childhood. ^^
Top-Left Corner: Chain Chomp
Begining with the giant scary dog-like chain ball on the first stage, the Chain Chomp.
Top-Right Corner: Bubba
Beware the water, Bubba, the giant fish with sunglasses, swallowing the player in 1 second with your big and deep mouth.
Bottom-Left Corner: Mad Piano
Seems to be a mere piano, but when you come closer, he chases you like a crazy and shows your deadly teeth!
Bottom-Right Corner: Unagi the Eel
The scariest enemy (in my opinion, of course). Man, what a giant monster, it's more creepy when he open his mouth, I used to keep away from this eel.
Well, time to remember your video-game's nightmares on N64. ^^