Aquatic Adventures: The oceanrider archetype medium cavalier gets the Orca (encoded as “Companion (Dolphin (Oceanrider Orca))”) as a mount option. It's listed but it bugs and doesn't work in general. The mount is created, but not linked, doens't get companion level and just doesn't work. I've attached a dummy cavalier character at level 3 so you can quickly check.
Also, when creating an Orca companion seperately, it fails to load after saving due to the Hold Breath special ability from de Dolphin (I think it is used as a template but the link is wonky).
Sources : Pathfinder RPG for Players, Advanced Class Guide, Aquatic Adventures.