Hippogriff is level 5 but costs as level 9

[Starfox]

yes, you can use it while mounted. result: you're not mounted any longer. being mounted doesn't change your spreed, you just use a move-action to command your mount to move its speed with you on it. of course, you could just not shift, and stay mounted that way.

This creates its own set of problems, as it is a Standard action to dismount.