Grimlet

Grimlet is an enemy found exclusively in Rusty Bucket Bay in Banjo-Kazooie. Three Grimlets populate the deck of the Rusty Bucket at all times, disguising themselves as three of the ship's six cowl ventilators. These stationary enemies can be hard to distinguish from the vents at a distance, but will growl and bare their foreboding teeth if Banjo tries to approach one. Don't get too close or the Grimlet will lunge and bite Banjo, sending him flying backwards, sometimes right off the deck and into the oily water below.

Grimlets are one of a few completely invincible enemies in the game; even the Wonderwing is ineffective against them. Naturally, since Grimlets cannot be killed and will attack anything that comes too close, it is impossible to enter them like the three normal cowl vents on the ship.