Door Keeper

Metroid:Zero Mission
In Metroid: Zero Mission (as well as other previous Metroid games), a Door Keeper is a simple creature that guards the entrance immediately before a boss. The creater is no more than a membrane and an eyeball that rest overtop of the door. It opens its eye periodically, to shoot a projectile at Samus at an attempt to keep her out. It is defeated with one missile to the eye.