The Rammerheads are enemies that can only be found in certain levels from Super Mario 3D World.
The Rammerheads are not hostile, as they just swim about in whatever pattern they are assigned. They are usually with other Rammerheads in this fashion. However, they running into one will damage you, so you must traverse around them in the levels they appear in.
Probably the most notable of their appearances in the one game they appeared in, Super Mario 3D World, was in the level Rammerhead Reef, where they take up most of the spotlight in the underground water levels. Their first appearance however, was in the level Captain Toad Makes A Splash, where there is one leaping over a wooden bridge that Captain Toad must walk over.