C. 1138
I have the Pathfinder Player Companion: Cohorts & Companions, and on page 9 it talks about creating magical beast cohorts. It has a table that lists both a Hippocampus and a Hippogriff as possible Bestial Cohorts.
The Monstrous Mount feat comes from the Pathfinder Campaign Setting: Inner Sea Combat, and on page 14 it lists both a Hippocampus and a Hippogriff as possible "Animal Companions".
I'm not that skilled with the PF rules, but I didn't see why a creature could not be either a cohort or an animal companion (monstrous mount). Those books seemed to have done it. I have no idea how you would go about creating your own starting package, so I was hoping someone here could figure it out.