Thanks to Synkai from WarcraftPets, one of the newest datamined pets from the beta, Fishy, was discovered to be a quest reward!
The quest that awards this pet is called Let Them Burn and seems to be at the end of a quest chain at the Jinyu Alliance quest hub in panda-land. I haven't found any horde equivalent of this quest yet, or a companion reward for the opposing faction, the Hozen. It's likely that horde will get their own version of the quest/pet in the future.
As Synkai mentions in the post linked above, this companion might have a death wish like our favorite suicidal pet, the Plump Turkey. Or it could be a bug. We'll have to wait and see which it is after the release of MoP!