Octoomba

An Electrogoomba, known as an Octoomba in Super Mario Galaxy 2, is an enemy who made its first appearance in Super Mario Galaxy. It is an alien very common in most galaxies, generally seen as an octopus-like creature. Despite its name, it hardly resembles a Goomba and their relation with them seems to be rather slim.

Super Mario Galaxy
They are seen in various parts of the game and headbutt Mario when he approaches. In some cases, if Mario destroys some, a secret may be unveiled, 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
Electrogoombas 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, similar to their pink sub-species (which still appear) and are called Octoombas. New subspecies were introduced as well such as an undead variant of them with the ability to float. There is also a green version that can shoot two stones instead of one.

Appearance
Electrogoombas are odd-looking creatures. They have smooth, blue skin with yellow spots, a puckered round mouth (like a Snifit's), glowing yellow eyes, and a small antenna on top of their head with a glowing yellow organ at the tip. Unlike a typical Goomba, the have no body. They attack with a headbutt, and only patrol a very small set area.

Sub-species
One sub-species is a pink color, and is somewhat smarter than the standard one. It attempts to run away from Mario, and will spit rocks at him if he gets too close. Another sub-species of Electrogoomba is gray, and taller and bigger then the other two. It is immobile, but can shoot projectiles at Mario. Some Gray Electrogoombas shoot either coconuts or fireballs, and others only shoot fireballs.

As the forms become more powerful they resemble more like octopi.

Super Mario Galaxy 2 features new yellow/green colored species, which apparently shoot multiple rocks in a row, as seen in Boulder Bowl Galaxy.

Trivia

 * Electrogoombas resemble both Octoroks and Deku Scrubs from many The Legend of Zelda games.
 * Electrogoombas have a similar appearance to the Bloopers found in Super Mario Sunshine.