Manta

The Manta or Manta Ray is an aquatic species found in various Mario games. They are peaceful creatures, much like Real Life Manta Rays. Some leave trails of regenerative rings behind them as they swim, and other can be used to surf above channels of water.

Super Mario 64
A Manta character first appears in Super Mario 64 and its remake, where it resides in the level Dire, Dire Docks, swimming in a circle around the whirlpool in the center of the entrance area. The Manta will leave a trail of shimmering rings as it swims around the whirlpool. If Mario can swim through five rings in a row, he'll be rewarded with a Power Star. Moreover, he can swim through a ring to regain lost oxygen. However, if Mario misses a ring, he has to start the five ring sequence all over again. If he swims through a ring correctly, a red number will appear. When Mario accomplishes the five ring task, the Power Star will appear at the top of the whirlpool. If Mario gets too close to the whirlpool, it will suck him in. Mario should also keep distance from the Manta's tail, as it can deal damage.

Mario Party 4
In Mario Party 4 there is a mini-game called Manta Rings in which a Manta releases rings while swimming.

New Super Mario Bros.
Manta was also in the Beta version of New Super Mario Bros. It is unknown whether or not they were enemies or just background scenery.

Super Mario Galaxy
A Manta named Ray appears in the game Super Mario Galaxy and takes the place of Bloopers in Super Mario Sunshine. Mario rides on his back during in the Loopdeeloop Galaxy and the Loopdeswoop Galaxy. Movement is controlled using the motion sensing capabilities of the Wii Remote instead of with the control stick as in Sunshine.

Also in Super Mario Galaxy, a similar character named Guppy challenges Mario to swim through eight rings in two separate galaxies.