Since we now have the option of monstrous mount, could I pick improved overrun for a hippogriff and go knocking flying foes down from the sky? Overrun says it knocks enemies prone if successful. Can a flying creature be knocked prone?
Is Flyby attack legal option for a flying animal companion in pfs?
If you can find the Flyby Attack feat in a non-Bestiary source, and that source lists it as a legal option, you can have your mount choose it (though I'm like 99% there isn't one).
Your first question is better asked over in the Rules Forum. Flying overruns have been around since Pathfinder became a thing. They aren't new.