Quote Originally Posted by Hor View Post
#showtooltip item:11122
/dismount [mounted]
/run SetMapToCurrentZone() local f,g,c,s="Pureblood Fire Hawk","Ironhoof Destroyer",GetCurrentMapContinent() s=(IsFlyableArea() and c>-1 and c<7) and f or g CastSpellByName(s) SetMacroSpell(GetRunningMacro(),s)

This macro when pressed uses the Pureblood Fire Hawk if in an area where you can fly, if not it will instead use the Ironhoof Destroyer. And, just so it doesn't constantly show a big black box with a red question mark I made it show the tooltip for Carrot on a Stick. Be aware, this macro is 254 characters long in it's current state. If you make any substitutions you'll need to either have an addon to extend macro length, or be careful about the length of the mount names you use.
You can cut 90% of that out with:
Code:
#showtooltip
/use [flyable] Pureblood Fire Hawk; Ironhoof Destroyer
Or flip it:
Code:
#showtooltip
/use [noflyable] Ironhoof Destroyer; Pureblood Fire Hawk
You can keep your dismount line in there if you want, but simply pressing the key again (when mounted) will dismount you.

http://wow.gamepedia.com/Macro_conditionals#flyable