Gadora

The Gadora is the "Eye Door", found in front of boss areas. There is one each directly in front of the rooms of Kraid, Phantoon, Draygon, and Ridley. There is also one two rooms before Mother Brain.

They can be defeated by shooting either one Super Missile or three Missiles into its open eye. After being hit by a missile it will close its eye briefly before reopening it.

After having its eye open for a short time, it will send out a burst of 3 orbs that will block shots, and will deal 4/2/1 damage to Samus if they hit her. These orbs will use the same palette as Samus' beam.

It's technically not an enemy, but a PLM (Post Load Modification, part of the room definition).

Gadora Glitch

If you kill a Gadora while one of its shots is on-screen, the game has a chance of crashing or lagging about 8 seconds later. Going through the door will prevent the crash, but pausing or using X-Ray will merely delay it. The cause of the crash is explained here.