The Fish

The Fish is an enemy in. It is considered to be the third boss of the game, but is actually the only boss which is optional to fight. It resembles a yellowish, carnivorous fish in an upright position and its purpose is to guard the Onyx Hook, an item which can transport Ninten to Magicant whenever he uses it. The Fish itself resides in the Magicant Underground, and the battle will more specifically initiate once Ninten has stepped on the stairs leading to the room which contains the Onyx Hook. It can be challenging to defeat because of its high stats and ability to use continuous attacks, and that PK Freeze and PK Fire is less effective on it.

In-game description
It seems to be guarding something.