Neptuna

Neptuna is the first boss on Desert Island, in The Deadly Tank of Neptuna. He was created by Baron Dante's magic that made a fish (a tuna, presumably) into a trident-wielding demon. He probably got his name from being turned from a fish and holding a trident, and just acting like the old Roman God of the sea Neptune. He is one of the hardest bosses to beat, and being underwater the whole time doesn't help.