Use:
Code:
#showtooltip
/cast [dragonriding] your favorite dragonriding mount; [flyable] your favorite flying mount; your favorite ground mount
Of course, you can add "[swimming] your favorite aquatic mount" but I wouldn't recommend because it'd mean you would have to first reach the shore before you can use that macro to summon your other mounts.
The one snag with that macro is that it doesn't work in instances such as Ruby Life Pools. But you can make a macro like this:
Code:
#showtooltip
/cast [modifier:alt] your favorite ground mount; [dragonriding] your favorite dragonriding mount; [flyable] your favorite flying mount; your favorite ground mount
With this macro, whenever you use it while pressing 'alt', it'll always summon your ground mount, regardless of where you are. You can replace "alt" for any other key, such as 'shift' or 'ctrl' if you prefer.