Champa's animal companion
Magic keeping him alive is from the school of abjuration, though that's also debatable. With a high enough roll or strong enough spell, it can be figured out that a wish spell (conjuration) was cast on a glass ball, so that there would always be a living pet fish in there. The spell created the fish, and then produced magic protection (most commonly seen as magic from the school of abjuration) to keep the fish alive "forever".
originally owned by a little kid whose mom cast a wish spell on the glass ball while it was in their hands. has ptsd from watching the kid fish and thus knows hes a fish.
never heard of xanathar