The Mantaman was an aquatic enemy with wing-like shapes on its back that fired projectiles at the player. Based on its animations, the Mantaman could be found with its wings over its body. When it saw the player, it would open up its wings and attack the player. Whether all of the Mantamen were supposed to spawn this way is unknown.
It is now confirmed as the Alpha was leaked, that the Mantaman can in fact get out of the water, like how the Fishman can. This doesn't change the Mantaman's animation however and looks quite bizarre - it was likely unfinished in this state.
There is no media that has proof of the Mantaman's existence. A restored version of this enemy can be found in Serious Sam Classics: Revolutions and will appear in Serious Sam Origins.