So, some more ideas.
Mobs: Sharks. They would be neutral mobs but when they are killed they would drop a shark fin which would be a potion item used for making a Dolphin's Grace potion. Penguins. Not-hostile mobs that could be tamed with fish.
New skins for dogs
Completely dyeable collars for dogs like leather armor. Made with one piece of leather. You need it to tame a dog.