Besides visual, mounts dont really have any differences. Why not give some mounts special abilities based on what they are?
Dog: Barks loudly, causing nearby enemies to flee briefly
Cat: not taking damage renders you invisible to enemies until jumping, attacking or taking damage
Horse (excluding slow sebastian): faster movement, and gives 2 extra jumps (cuz horses can jump far)
Bull: dash forward knocking aside enemies
Manta: like turtles, doubles as a boat; can also dive
Caterpillar and spider: can climb walls (not ceilings)
Im sure other people can come up with more ideas, feel free to add
Dog: Barks loudly, causing nearby enemies to flee briefly
Cat: not taking damage renders you invisible to enemies until jumping, attacking or taking damage
Horse (excluding slow sebastian): faster movement, and gives 2 extra jumps (cuz horses can jump far)
Bull: dash forward knocking aside enemies
Manta: like turtles, doubles as a boat; can also dive
Caterpillar and spider: can climb walls (not ceilings)
Im sure other people can come up with more ideas, feel free to add