Double Tail Male Betta (Betta splendens)

The double tail gene doesn't just split the caudal fin into two lobes, it widens the dorsal fin and shortens the body slightly too, since the same mutation affects fin development broadly rather than just at the tail. It's recessive and notoriously hard to breed true, which is part of why a clean double tail still turns heads in the hobby. One male per tank, that part never changes.

Care level: Easy · Temperament: Aggressive toward other bettas · Max size: ~3" · Min tank: 5 gal · Diet: Carnivore, betta pellets + frozen