RandomMount is a lightweight addon that attempts to emulate Retail's "Summon Random Favorite Mount" feature for WOTLK Classic.
In the interface options -> AddOns -> RandomMount you can select which mounts you want /randommount to pick from. It'll automatically select between flying and ground mounts depending on whether or not you're in a flyable zone.
Select and save your favorite mounts and summon them with /randommount