yet to me mounts should have a purpose.
Besides getting you faster from A-B and maybe some extra bag space, what purpose are you thinking off?
some would be faster, some for multiple people, some for pulling light loads, some for heavy loads, some to scale cliff sides, etc.
These ideas for specific mounts having specific benefits would tie in well with thw horizontal progression. Have Scout Mounts, Cargo Mounts, Terrain Mounts and so on.