I've got a mount macro in which I follow a theme. I try to include mounts that (1) are the best representative of the expansion they came from (that are in my collection) and (2) are compact enough I can see around them while flying. (It's Horde-specific.)
Right now, I have:
The Burning Crusade: Grand Wyvern
Wrath of the Lich King: suggestion needed
Cataclysm: suggestion needed
Mists of Pandaria: Imperial Quilen
Warlords of Draenor: Grinning Reaver
Legion: suggestion needed
(I've also got the Hearthsteed and the Jeweled Onyx Panther, just because. And, on Alt, I have (ground-only) Mechano-Hog and (flying allowed) Sandstone Drake and Obsidian Nightwing as "bring a buddy along" alternatives.)
Note that the mount doesn't have to be from that expansion. It just has to FEEL like it's from the expansion. And it has to be small (so I can see around it).