Griffon (from Cook)
No. App: 0 (2d8)
HD: 7
AC: 5
AL: N
Move: 120' (40')
-Fly: 360' (120')
Att: 3 (2 claws/1 bite)
Damage: 1d4/1d4/2d8
Save: F4
Morale: 8
The griffon is a pretty tough cookie. Its AC isn't that great, but it isn't awful. It gets three attacks per round and the beak attack does some respectable damage, too. What's interesting about the griffon (apart from the flying thing) is its morale score (and using it).
"Its favorite prey is horses. When within 120' of horses a griffon must pass a morale check or attack immediately."
The young can be tamed (like Pegasi), but they still make morale tests near horses. This is a savage mount, and one I could easily see ridden by some chaotic warlord.
Hippogriff (from Cook)
No. App: 0 (2d8)
HD: 3+1
AC: 5
AL: N
Move: 180' (60')
-Fly: 360' (120')
Att: 3 (2 claws/1 bite)
Damage: 1d6/1d6/1d10
Save: F2
Morale: 8
No, I wasn't going to use Buckbeak!
These creatures are a bit less robust than griffons (lower HD), but I suppose that's the horse vs lion parts thing. The notes about taming them does not explicitly state you must get a young one, but that's GM discretion, to be sure. "They will usually attack pegasi, who are their natural enemies." as I mentioned previously. Surely there is some plot fodder there.
Hippogriffs and Griffons are of animal intelligence, so that probably makes more complex training difficult. A flying mount could be a great status symbol, not to mention things like adding aerial combat to a game. Imagine being able to couch a lance and charge a dragon in the air!