The below commands are available to all players on your server, providing you own the horse. You must be sitting on the horse for these commands to work.
/horse trustlist If you own the horse, this will print a list of trusted players for this specific horse.
/horse trust player Replace 'player' with the player name you wish to trust. This will grant them access to ride and lead your horse, but not harm it.
/horse untrust player Replace 'player' with the player name you wish to remove trust access for. This will remove any permissions they had to ride and lead your horse.
/horse transfer player This will transfer ownership of the horse to the player you specify in the command.
Permissions
The below permissions are ideal for server mods or staff allowing them to manage player owned horses on your server.
shantek.horseguard.ride
Players with this permission will be able to ride and lead any horse, ignoring any ownership or trust settings.
shantek.horseguard.damage
Players with this permission will be able to harm any horse, ignoring any ownership or trust settings.