Octoomba

An Octoomba, once called Electrogoomba in Prima's Official Guide, is an enemy that first appeared in Super Mario Galaxy. It is an alien very common in most galaxies, and bears a slight resemblance to Goombas and octopi, both of which contribute to its name.

Super Mario Galaxy
They are seen in various parts of the game and headbutt Mario when he approaches. In some cases, Mario must defeat several to progress, as seen on several Airships in the game, where Mario had to eliminate two of them to make a bridge appear. They can easily be squashed. When jumped on, a Coin is always left by them. When a Star Spin is used to defeat them, they leave three Star Bits.

Super Mario Galaxy 2
Octoombas re-appear in the sequel, Super Mario Galaxy 2. Unlike in the prequel, they won't headbutt Mario, but rather, they will shoot boulders at Mario, and their name is finally revealed to be Octoombas. New sub-species were also introduced (outlined in "Sub-species", below).

Appearance
Octoombas are odd-looking creatures. They have smooth, blue skin with yellow spots, a puckered round mouth, glowing yellow eyes, and a small antenna on top of their head with a glowing yellow organ at the tip. They attack with a headbutt or by spitting rocks, and only patrol a very small set area.

Sub-species
A pink sub-species is known as an Octoguy, and is somewhat smarter than the standard one. It attempts to run away from Mario if he gets too close and will spit rocks at him once at range.

Another sub-species of Octoomba is gray, and taller and bigger then the other two, called an Octopus. It is immobile, but can shoot projectiles at Mario. Some Octopuses shoot either coconuts or fireballs, and others only shoot fireballs, similar to Birdo in her Super Mario Bros. 2 appearance.

Super Mario Galaxy 2 introduces the Elite Octoomba, a green Octoomba with the ability to shoot two boulders, one after the other.

Trivia

 * Octoombas resemble both Octoroks and Deku Scrubs from many The Legend of Zelda games.
 * Octoombas have a similar appearance to the Bloopers found in Super Mario Sunshine.
 * Their voice sounds similar to that of the Shroobs.