Manta

The Manta or Manta Ray is an aquatic species found in various Mario games. They are peaceful creatures, much like manta rays on Earth. Some leave trails of regenerative rings behind them as they swim, and others 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.

New Super Mario Bros.
Manta was also in the beta version of New Super Mario Bros.. It is unknown whether they were enemies or just background scenery.

New Super Mario Bros. Wii
Types of Mantas called Jumbo Rays appear in New Super Mario Bros. Wii. They are green and wear a pair of water goggles. They can fly, so Mario and company can stand on them as platforms for a short time. Their appearance and manner of aiding the characters to pass some obstacles are rather similar to those of Dolphins from Super Mario World.

Mario Party 4
In Mario Party 4, there is a minigame called Manta Rings in which a Manta releases golden rings while swimming. These rings give the players three points unlike the usual blue ones, which give out one point.

Mario Kart 8
A single Manta appears in Mario Kart 8 on the Dolphin Shoals course. It looks like a blue Jumbo Ray.