Page 1196 — The Over-Under

In 4th Edition, you could play as a Revenant, and one of their features was the "Past Life," where you selected the race you inhabited among the living and thus qualified for any feats, paths, or destinies requiring it. If one were to expand the seapony lore and say all kinds of races fled underwater and assimilated into the welcoming fins of Queen Novo, then I imagine you could turn the seapony's former heritage into a similar kind of crunch.

Pinkie Pie: GAAAAAAAHHHH!!
DM: …What?
Pinkie Pie: I can't decide! I wanna be a seapony so bad, but I also wanna be a hippogriff and fly and have opposable claws and all that jazz! HOW CAN I CHOOOOOOSE??
DM: …Well… in this specific case, you might not have to.
Pinkie Pie: Bwuh?
DM: I have to tackle the problem of sea-creatures that can campaign on land anyway. It's in their backstory that seaponies were previously another race that was driven underwater by the tyrannical Storm King, and they can switch forms at will. That prior race being hippogriffs would make things interesting in a number of ways. A duality between sea and sky. So you'd basically have a bonus action that allows you to swap forms, one with a fly speed and one with a swim speed.
Pinkie Pie: WOOHOO! Hipposeaponygriff it is!