Rule of the Day 04/06/2011
This is not directly a written rule but rather a change from 7th edition, though I think it is an important one.
pg. 70 - Fly
Pg. 83 - Monstrous Cavalry
Pg. 85 - Monstrous Beasts
pg. 97 - Characters
The above pages contain all of the rules needed to understand this change.
In 7th edition, there was a rule that characters could never join units while mounted on a flying creature.
There are lots of rules now dictating what units a character can & can't join, but being on a flying monstrous beast is no longer one of them.
Can't Join Monsters
Can't join Flying Units
Can't Join Chariot Units
Can't join Swarms
Can't Join War Machines
Can't Join units when riding a monster or if he is a monster
Who benefit from this? Elves that can ride great eagles and Dark Pegasi. Humans can ride pegasi so could also benefit. Lizards that can ride terradons.
I think that covers it. Full Credit where credit is due here - I didn't realize this change until I read a battle report on TheWarhammerForum by Ben Curry from the Bad Dice Podcast where he used this tactic.
He put the unkillable DE Lord on a Dark Peg in a unit of Spears, also with the crown of command. This guy was able to charge out and basically lock down any unit with Stubborn 10 (likely re-rollable) or kill mages / war machines, etc... Lots of utility here. He doesn't get a Look out Sir, but you actually would prefer enemies shoot him instead of the unit (given his un-killablyness).
This is probably the single best of this unit as the magic items available make the elf hard (if not impossible) to kill. But don't let that stop you from trying it!
Just another tool to consider when building your list.