Fish Bone

Fishbones are swimming skeletons of fish found in the game Super Mario World. They always swim slowly in a straight line, heading to the left. Fishbones can only be defeated by Cape Mario or Cape Luigi's spin attack. It is unknown what type of fish these skeletons were originally. 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 were the only strictly aquatic dungeon enemies in the game, though terrestrial enemies were also found underwater in these levels.

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 position 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. 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 Wet Bones in the Prima game guide.

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 kill a Junker Can, a Fishbone may attempt to attack the plumbers.

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 Land 2: 6 Golden Coins
Fishbones make an appearance in Super Mario Land 2: 6 Golden Coins. They are found in the third level of the Macro Zone and the final stage of the Turtle Zone. Only a Starman can defeat them.

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.

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

Trivia

 * In the 3DS's Fishing AR Minigame, after lightning strikes the water, the player can fish out Fishbones.