Accidentally Scary Enemies

Redeads and wallmasters from Ocarina of Time, the Eel from super Mario 64, That giant Dionaga from Star Wars: Shadows of the Empire, Sinistar, and The Giant Octopus from Echo the Dolphin all terrified me when I was younger.  My friend's sister shrieked when she got seen by a guard in Metal Gear.

The Chained up chomp thingy in the first level of Mario 64 . . . I thought it was alive XD