Manta Rays are a mob added as part of the Mo' Creatures mod. They are sea dwelling creatures, and can be found in most bodies of water, much like squids.
Manta Rays are a peaceful mob. They will continue to wander unless provoked.
They can be ridden by right clicking them. When ridden, they will shove the player off after a few seconds, but if you try enough they will eventually allow you ride them. You use same controls as a boat to "steer" it, with the exception of the spacebar, which will allow you to ascend. Move around to descend. It has been noted that it can get tired and will go slower. Also, if the ray has a block directly below it, it will go slower. If on land it will suffocate and die.
Manta Rays have the ability to inflict the "poison effect" like jellyfish. It lasts for 5-15 seconds, depending on the size of the ray.