Hey,
I searched other posts to do with the 100 mount achievement and found nothing with this, so I'll go ahead and post it
WoWCollect.com - Simple Checklist
This link will help you keep an easy record with what mounts you already have, what mounts you need to get, how many you have, how many you need, where to get them, etc, etc.
Moreover, there is a seperate tool for both tabards and pets!
Happy hunting!