Part-lion, part-bird of prey, their hooked beaks easily severing the limbs of those who get to close
The Imperial Griffon is the rarest of the Empire mounts, being available only to Karl Franz, Boris Todbringer, Amber Wizards, and the lord class General of the Empire. It is very difficult to mass them in an army; this is good, however, as the Griffon is also by far the strongest of the mounts.
There are reasons to take the Imperial Pegasus--the Pegasus is noticeably faster, offers a better charge bonus, and gives no reduction to Melee Attack and Defense. However, the Griffon has better combat stats, giving a massive boost to Weapon Strength along with a slightly higher boost to Leadership, and while not as high as the Pegasus' boosts, its other stats increases are significant.
The real draw of the Griffon, however, are its abilities and attributes. Apart from Flying, the Griffon causes both Fear and Terror, and has an aura hex known as Bloodroar. Fear and Bloodroar combined means units trying to fight within 30 meters of the Griffon take a whopping -16 penalty to their Leadership, and thanks to Terror, the threshold for them breaking and running is made much easier to meet. Terror routs are not as long-lasting as normal routs, but they will still throw a formation into chaos and give you gaps to exploit. Finally, since the Griffon causes Fear and Terror, its rider is immune to both of those effects as well, which makes any Lord or Amber Wizard riding on them much harder to break in battle, and perfectly suited for fighting the enemy's big monsters.
Griffons are excellent picks for your dedicated melee combat Lords, which fortunately describes all of the Lords who can get them (and while Amber Wizards are still casters and should not be considered the equivalent to a fully kitted Melee Lord, their buffs, debuffs, and summons mean they can dive into the fray much more readily than any of their fellows). A Lord on a Griffon is a highly flexible unit, able to charge into and shatter an enemy front line, skip right over it to massacre their missile troops and artillery, or--possibly most effective--hunt down enemy Lords with a speed that only another flying unit can match.