Though I'm looking for a way to have two macro's from mounted on my action bar. One will cast a random 100% ground mount and another will cast a 280% fly mount.
As I have it right now, it only summons a random ground mount and I haven't got a randomizer for my flying mounts. This because there are situations that I want to ride a random ground mount in an area where it is possible to fly.
Try Squire. You can configure as one button to cast a random ground mount or flying mount when possible, but if you press a modifier (control, shift, alt) it will cast a ground mount despite you're in a flyable area.
I believe mounted does the same thing. I have the mounted macro bound to CTRL-R. When I land in dalaran at krasus' landing and want a ground mount, I press CTRL-ALT-R and am all set to go.
I didn't find that option in Mounted. But maybe I have just overlooked it. I'll try the addon suggested above first, since that's the one currently installed. But if I don't like it, I'll have a look if I can find that option you talk about.