Urchin

Urchins are blue sea urchin enemies in the Mario series.

Super Mario World
Urchins are found only in Forest of Illusion 2. Urchins can only be defeated by a grab block at it.

Urchins aren't affected by fireballs and they are too big to be eaten by Yoshi, but Yoshi can bounce on them.

Mario must time his movements to avoid an Urchin's spikes.

In the Mario Advanced 2, Luigi on Yoshi can defeat it by spitting out a Blurp.

Super Mario World Cartoon
Urchins made a small appearance in Mama Luigi. Luigi and Yoshi get past a Rip Van Fish and they ignored them.

Nintendo Adventure Books
Urchins only appeared in Brain Drain in a large fish tank by Luigi after Iggy takes out the WMUSH radio station.

Nintendo Power
Urchins only appeared in Super Mario Adventures and several seem to act as the guards in the moat surrounding the Koopalings tower.

Yoshi Touch & Go
Urchins were only features in Balloon Trip and can be thought in the beta where they float in the skies and take Baby Mario's balloons away if he's about to touch them and they cannot be trapped by bubbles.

Super Princess Peach
Urchins are found in Subrella levels, where Princess Peach fights them instead of Mario in the game. They move back and forth, are invisible at all, like the Torpedo Ted.

Super Mario Galaxy
Urchins are found only in Beach Bowl and Deep Dark Galaxy. Urchins stay on land instead of swimming. They roll to chase Mario when they see him. There are also Urchins at Freezeflame Galaxy.They do the same move underwater. In the fire are of Freezeflame, there are red Urchins and act the same as the normal ones. Mario can only defeat them if he throws a fire ball when he is Fire Mario or tricking them to enter the lava. The underwater versions release six Star Bits when dead and the underwater ones can be defeated by a shell.

New Super Mario Bros. Wii
Urchins are found only underwater, along with a sub-species of an Urchin called Mega Urchin. When Ice or Penguin Mario freezes them, they float to the top of the water or get knocked down instantly underwater. It can block underwater Warp Pipes that shoot out jet streams and can be used for Mario's disadvantage or advantage, depending on the situation. They can be killed by a Fire Mario's fireballs, throwing a Koopa Shell or when Mario has a Starman on him.

Super Mario Galaxy 2
Urchins made their latest appearance in Super Mario Galaxy 2. They behave like in the first game and are found only in Starshine Beach and Slimy Spring Galaxy. Red Urchins only appear in Battle Belt Galaxy.

Trivia
In Super Paper Mario, one Sammer Guy named Urchin Lung is the reference to an Urchin.

Hacking into Super Mario RPG reveals that game has a beta item that it would be a tiny Urchin, likely of some kind of item.

In the Trading Card version of Super Mario Galaxy, the Urchin states that this enemy is a blue, spiky fish, but they seem to be purple.