Is there anything about having a medium character ride a medium mount? Can it be done? Are there penalties? This is assuming that it is withing the weight limits of the mount.
Nothing in RAW against it, so long as you+your gear+mounts gear doesn't go over the mount's maximum load.
Get ready for an awful lot of debate though. Many say it may not be RAW but it's RAI to have a mount one size larger.
A human riding a pony would have to cross his legs, is all I'm saying.
The search feature is your friend. Use it to find threads like
This one
or this one
Thanks guess I need to work on my searching abilities