Pathfinder RPG Inner Sea Compbat has a Ranger Archetype called Sable Company Marine. That archetype permits the ranger to take a Hippogriff as a monstrous companion. The hippogriff option does not appear in the Add Companion pop-up.
Possibly related to this issue is the Beastmaster archetype from APG that should allow any AC from the druid list rather than just the limimted ranger list. Assuming that this once worked, perhaps what broke this is also preventing Sable Company Marine from working.