Urchin

Urchins are slow, blue, spiked sea creatures that have appeared as enemies throughout the Mario series.

Super Mario World
In Super Mario World, Urchins move between tunnels in Forest of Illusion 2. Yoshi can't eat them, but he can bounce on them. They can be defeated by Grab Blocks. Players must time their movements carefully as to avoid an Urchin's spikes.

The Urchins in Super Mario World: Super Mario Advance 2 can be defeated by Luigi riding on Yoshi and swallowing a Blurp and spitting it at the Urchin.

Super Mario World TV series
Several Urchins make a small appearance in the Super Mario World episode "Mama Luigi". In the episode, Luigi and Yoshi, while being chased through an underwater tunnel by a Rip Van Fish, almost swim directly into several Urchins. Luckily, Luigi and Yoshi manage to avoid bumping into these Urchins.

Super Mario Adventures
In Nintendo Power's Super Mario Adventures comic, several Urchins are seen to act as guards in the moat surrounding the Koopalings tower.

Yoshi Touch & Go
Though they were seemingly going to make an unexpected return in Yoshi Touch & Go, Urchins were not seen in the final game. They were, however, featured in the Balloon Trip tech demo that can be thought of as a Yoshi Touch & Go beta, where they would (perhaps strangely) float in the skies and take one of Baby Mario's balloons away if he was to touch one of them; they could also never be trapped in bubbles, making them some of the more challenging enemies featured in the demo. By the final game's release, these Urchins were redesigned as the plant-like Briers.

Super Princess Peach
Urchins also make an appearance in Super Princess Peach. They appeared in the Subrella sections of the game. They would move back and forth, and could not be defeated.

Super Mario Galaxy
Urchins appear in Super Mario Galaxy in the Beach Bowl Galaxy and the Deep Dark Galaxy. However, they do not swim around in the water, instead they will stay on the sea bottom, and when they see Mario, they start chasing him by rolling towards him. Urchins also appear on land, in the Freezeflame Galaxy. They act exactly the same as they do underwater. There are also red Urchins, to go along with the fire area of the stage, they will act exactly the same as the normal Urchins, and can be killed if the player tricks them into entering the lava or if they are hit by a fireball shot by Fire Mario or Fire Luigi. The underwater versions can be destroyed with a shell. They release six Star Bits when defeated.

New Super Mario Bros. Wii
Urchins appear in New Super Mario Bros. Wii along with a bigger version of them called Mega Urchins. Urchins start floating up and down like their Super Mario World incarnation, but when frozen with an Ice Flower or Penguin Suit they will float briefly on the water before breaking free and getting knocked out underwater permanently. In this state, the Urchin can block underwater Warp Pipes that spray out jet-streams, which can be used to the player's advantage or disadvantage, depending on the situation. They could also be defeated with a fireball, a Koopa Shell or a Starman.

Super Mario Galaxy 2
Urchins reappear in Super Mario Galaxy 2, behaving the same way as in the prequel. They appear in the Starshine Beach Galaxy and in the Slimy Spring Galaxy. Red Urchins also appear in this game in the Battle Belt Galaxy.

Trivia

 * The Sammer Guy "Urchin Lung" from Super Paper Mario may be a reference to an Urchin.
 * Hacking into Super Mario RPG: Legend of the Seven Stars reveals that game was initially planned to feature what would appear to be a tiny Urchin, likely as some kind of item.