The head is a cannon. These beasts are horrific sights to sailors out at sea, as they take sadistic pleasure in sinking ships with a merciless barrage of cannonballs.
The Seagun is the Stage 3 power form of the Seabreeze group.
Seaguns were removed from the game before it was released, so although their data still exists, they do not appear anywhere.
A Seagun should evolve from Seavolver, but it is disabled in the alpha version. It does not evolve further.