How about the beta fish starts as a solid Water type, but the males evolve into Water/Fighting types and the females evolve into Water/Psychic types?
Believe it or not, female bettas are pretty aggresive as well. Still a good idea! Why psychic?
I was thinking both male and female look the same but then when it evolves if it were a male, the tail would be longer and prettier and if it were a female, it would be plane and short. Still, it would be cool if they had different forms (like northend/southend...)
Female : http://knol.google.com/k/-/-/2u2wxs470ayns/st1qh3/betta05g.jpg
Male : http://www.bettafish.name/Betta_Fish.jpg