Mount Toggle will put you on your mount if you're not mounted and dismount you if you are mounted. Mount Toggle will scan through the Partner Bag first, the Item Shop backpack second and finally the backpack for any mounts that you have and use the first one it finds.
To use the addon put the following line in a macro or type /mtmg into chat to generate the macro.
To test if the Mount Toggle addon is loaded in a function or addon use
if MountToggleLoaded then
v1.12 Added in the ability to use a Horse Rental Ticket if no mounts are found
v1.11 Changed a string to work for all rented mounts.
v1.1 Streamlined how items are searched to determine if it's a mount.
v1.0 Working as intended