Once you get more than a handful of different mounts in WoW, summoning a random one is a fun idea that creates some variety when you are playing. It's actually very easy to summon a random mount, and you don't even need an add-on or a macro to achieve this.
Firstly open up your mount journal (Shift + P) to see all your mount. Now select all the mounts you want to be summoned and right click on them. Click on "Set Favorite" for all of these. To truly summon a random mount, add all as favorites.
Now see the icon in the top right corner? Drag this to your action bar, and any time you click on this any random "Favorite" mount is summoned!