http://www.wowinterface.com/download...-MountSpy.html
or
https://mods.curse.com/addons/wow/mountspy
Just click on any mounted player and MountSpy will tell you the name of the mount, how to get it, and whether or not your character has it. If it's from an achievement, it'll give a link to that achievement so you can see how close you are to getting it.
I built this mod because I'm crazy about collecting mounts, and I grew tired of clicking on a player and mousing over their spells to find out what they're riding and then having to tab-out to a browser to find out where to get it. It's also frustrating when you see a cool mount but the player has like 10 buffs and moves out of range before you can mouseover the right one.
There's also a Mount Match feature, which is a hoot. Whenever you're sitting around waiting in a group for a rare spawn to show up there's always mount guys showing off, and Mount Match puts you on the same mount as your target instantly (if you have collected it, of course).
Upcoming features include a mount wishlist so you can keep track of the ones you want most (and it will include where to get it, who drops it/sells it, etc.).