Neptuna

Neptuna is the first boss on Desert Island, in The Deadly Tank of Neptuna.

Biology
Neptuna appears to be an anthropogenic fish, excepting the fact that he has no legs but a fin instead.

Origins
He was created by Baron Dante's magic that made a fish (a tuna, presumably) into a trident-wielding demon.

Name Origin
He probably got his name from being turned from a Tuna fish and holding a trident, and just acting like the old Roman God of the sea Neptune.

Method of defeating
He is one of the hardest bosses to beat, and being underwater the whole time doesn't help. Croc must wait until Neptuna has shot damaging beams from his trident before advancing and attacking him with a tail spin.