@newmount is a test command, it'll no longer be available after tests finish so you should not make it availabe to your players.
As it's a script as many others you can just apply it to an item script so onclick the user will mount or dismount or you can just use:
12622,Reins_Of_Mount,Reins Of Mount,11,0,,10,,,,,0xFFFFFFFF,7,2,,,,,,{ setmounting(); },{},{}
That already is in the database from rA, it's the same as @newmount.
I don't know if there's a OFF button as it's a client icon and I have not seen this in any server/client.