Fish Bone

Fishbones (also known as Wet Bones) are swimming skeletons of Cheep-Cheeps which first appears in the game Super Mario World. They may also be related to some of the other kinds of undead fish appearing in the Mario series.

Super Mario World
Fishbones are found in the submerged Vanilla Fortress and the underwater room of Bowser's Castle. They are the only strictly aquatic dungeon enemies in the game, though terrestrial enemies are also found underwater in these levels. Their behavior is simple, traveling in straight lines across the screen in a jerky, darting motion, usually in small groups. Fishbones can be defeated by Cape Mario or Cape Luigi's spin attack, but only in the Super Mario Advance 2 release, and are otherwise invincible.

Super Mario Galaxy
In Super Mario Galaxy, Fishbones appear in the Bonefin Galaxy during Mario's boss battle with Kingfin. In this game, they have glowing red eyes, and attack in a way that's very similar to Torpedo Teds; by homing in on Mario and striking him to cause damage. They can be destroyed when touching the surface, like the real-world fishes. After the player has used a Green Shell or a Red Shell to dole out some damage to Kingfin, the skeletal shark summons the voracious fish to aid him in attacking Mario. When the player has hit Kingfin two more times, he will summon a lot more Fishbones, which attack the player until the end of the battle, after which they disappear.

New Super Mario Bros. Wii
Fishbones appear in various water levels in New Super Mario Bros. Wii. When a Fishbone detects one of the characters, it will lock on to them (as their eyes turn red) then launch themselves in the direction of the character. If it misses, it will crash and break apart in a similar manner to that of Dry Bones. However, once they break apart, they are permanently defeated. They even break apart if they hit another Fishbone. But if the player jumps out of the water, the Fishbone will cease its rampage towards the player. In dark caverns, their eyes light up, although the illuminated area around them is much smaller than that provided by Bulbers and thus far less useful. They are referred to as Wet Bones in the Prima Games guide. A method for dodging a Fishbone is to walk on the floor of the water's level and then duck once spotted. The Fishbone's eyes will turn from red to green and no longer chase the player. Fishbones appear in the World 4 Castle and the World 5 Ghost House. Also, in the battle against Wendy O. Koopa in the World 4 Castle, various Fishbones can be seen swimming in a water tank in the background.

New Super Mario Bros. 2
Fishbones re-appear in various castles and fortresses in New Super Mario Bros. 2 where they keep their design from New Super Mario Bros. Wii. They act like they did in previous installments.

New Super Mario Bros. U
Fishbones reappear in New Super Mario Bros. U, having the same role from previous installments. They appear in water levels.

Mario Kart series
Fishbones make their first Mario Kart appearance in Mario Kart 7 and take their design from New Super Mario Bros. Wii. They are found in Wario Shipyard, where they swim in schools or sometimes alone. If any drivers collide into Fishbones, the former will be pushed back.

Fishbones will also appear in Mario Kart 8 in the underwater section of an unnamed haunted mansion-themed course. They serve as obstacles just like in Mario Kart 7.

Super Mario World television series
Fishbones, though never called as such, made several small appearances in the Super Mario World cartoon.

A Fishbone first appeared in the episode "The Wheel Thing", where one is used by Yoshi to cut a log into wheels to be used in Mario's car factory. In "The Night Before Cave Christmas", both Mario and Oogtar use a Fishbone to cut-down several Christmas trees.

In "Rock TV" and "Mama Luigi", two extremely large Fishbones are shown to be decorations in Bowser's throne room.

Super Mario Adventures
In the Super Mario Adventures comic, Fishbones, along with several other creatures such as Urchins and Porcupuffers, are shown to act as guards/obstacles in the moat surrounding the Koopalings' tower.

Mario Bros. (Super Mario Advance)
Fishbones, known as "Fish Skeletons", are a useless item that Mario sometimes receives after being thrown in the trash can in the Mario Bros. battle mode of the Super Mario Advance series.

Mario & Luigi: Bowser's Inside Story
Fishbones make another appearance in Mario & Luigi: Bowser's Inside Story during the battle with Junker. When Mario and Luigi manage to defeat a Junker Can, a Fishbone may attempt to attack the plumbers. It attacks by orbiting around one of the two and eventually ramming them, though it can be stopped by counterattacking with the hammer before it does.

Mario Party 9
The Mario Party debut for Fishbones (not counting their cameo in Mario Party 4) is in Mario Party 9. They appear once again with their New Super Mario Bros. Wii appearance. Fishbones only appear during the Blooper Beach Captain Event; when the player lands on an Unlucky Space on this part of the board, a group of Fishbones attacks them and steals five of their ten Mini Stars.

Paper Mario: Sticker Star
Fishbones make an appearance in Paper Mario: Sticker Star. They appear in Jungle Rapids, leaping in the water and attempting to destroy the raft Mario uses to cross through the river. They can be removed using the hammer.

Trivia

 * In the Nintendo 3DS's Fishing AR minigame, after lightning strikes the water, the player can fish out Fishbones (known as Fish Bones in the game).
 * In the minigame Mushroom Medic of Mario Party 4, there is a poster in the background with a Cheep-Cheep and its Fishbone X-ray, confirming Fishbones are undead Cheep-Cheeps.
 * Fishbone is a pun on fish and wishbone.
 * A similar enemy called Shell Missile from Donkey Kong Country 3: Dixie Kong's Double Trouble! act almost like fishbones.
 * They are similar in appearance to the Skullfish enemy from the The Legend of Zelda series.