Tamarin Daddy Daycare 14 1, 11:29am

http://en.wikipedia.org/wiki/Sex_change#In_animals
There's some nice ones you could still do. The German version also mentions a fish, that may even get pregnant after changing it's sex from female to male. Probably Loki is real, afterall
Weirdly the English article about the fish doesn't mention that fact: http://en.wikipedia.org/wiki/Green_swordtail