What if Fundy is a Pooka? It would make sense considering his mother is a fish and his dad is a human, and this somehow resulted in a fox. If Sally was a Pooka in the form of a fish, it's possible that Fundy is half Pooka.

(BTW if you don't know what a Pooka is, it's a type of fairy that often takes the form of a large humanoid animal. Mostly depicted as rabbits, but can be other animals as well.)