With single button you can ride the selected mount (flying or ground or even swimming, depending on type of zone and your knowledge of skill) or, if you're mounted or riding a vehicle you can dismount/exit vehicle.
Based on the original code of SimpleMount which seems to be not updated.
Just make a macro:
/usemount Flying mount here, Non-flying mount here
Added Deepholm support
Added Vashj'ir support — no changes to your macro if you have Sea turtle it will be auto-casted, if you have only Seahorse —*it will be auto-casted, if you don't have anything —*go do your quests, man!
More checking for different zones
Added checking if old world flying spell is known