WarcraftMounts, the classic mount search site, has implemented a section to list and track your mount collections!
WarcraftPets has long had such a system, but while I think Pets has a better collection layout (smaller icons, thus easier to peruse) and even a ranking system, the Mounts collections have something I've wanted on the Pets for a long time -- collections per character without having to create a second account.
Also, considering they just released this collection system on Monday, I'm sure we'll see plenty of upgrades eventually. This is the unveiling and there are sure to be improvements to be made. If you have any thoughts or find any bugs, please mention them to the site admins in a polite and respectful way. :)
I'm just tickled pink. XD As my mom would say.
I'm working on Birdfall and Dustfire, but I'll need to log in to see exactly what they have. Please link your collections in the comments when you get them done!! :D