Steven

A goldfish belonging to Magnus Burnsides. His full name is Lord Steven Q. Fletcher Esquire the Goldfish III (maybe).

Steven lives in a magical snowglobe-sized glass ball, which was inside the Mystery Bag Magnus bought at Fantasy Costco in Episode 17: Lunar Interlude I: Carnival Chaos.

In Episode 18, Merle accidentally gains command over Steven by casting Channel Divinity: Charm Animals and Plants. He makes him poop... And then the poop immediately dissolves in the water. It's a magic ball, and it dissolves the poop, and then the poop turns back into air, so stop asking about where the poop goes! Griffin clarifies.

Steven has his own null suit that he wears in Lucas's crystalizing laboratory during Chapter 4: The Crystal Kingdom.