I really want to see fighting with mounts and multiplayer mounts. I only know one MMORPG with both of those features.(i dont know one which has only one of them.) They are really fun.
Here is an idea how it could work:
A player can own a mount. You can ride free, but not attack while riding. If you try, the mount throw him down and run a short distant away.
If you want to fight with it, you must train it for war. Then the mount becomes similar to a hired NPC.(It has health, skills, max weigth, stamina, etc) But if it is killed and not ressurrected in time,it is permadead. Also you can buy equipment for it, e.g. armour for mount, armoured sattle.
The mount can have melee attack or a short ranged shot/magic, not both.
Ranged attacks need ammo and the rider can only attck with the mount\'s attack. If the mount has a melee attack, the rider can choose between his and the mount\'s attacks. (On multiplayer mounts, the rider can only use the mounts attack indepent from type. And not all mounts must have an attack.)
A hard hit at the rider, can make him fall off.
For balance ranged attacking mounts should not flying and rider\'s ranged/magic attacks should have a malus.(Otherwise every nonranged is an easy target.)
Some examples to illustrate it a bit more (i know those animals will pretty sure not in PS, but perhaps something alike) :
Horses: fast, mid health, melee attack front, back. You can buy saddles, body armour, and chariots. Saddles influence you too.(harder to make you fall off, give armour to you, but better saddle makes horse slower, costs stamina) Body armour protects the horse better with same downs as saddles.
Chariots comes in one-horse and two-horse variants. Both make the horse slower, cost the attack, but give protection to rider and allow one(one-horse)/two(two-horse) fighting passangers(only their own attacks). Chariots can be destroyed without killing the horse and horse can be killed without destroing the chariot. Chariot can only combined with light body armour, not with heavy or saddles.
Elephant: slow, good health,melee attack front. You can buy weapons, body armour and cabines. Melee attacker need a long weapon or a jump attack against the rider. Weapons are for the tusk and make the attack stronger. Body armor same as for horse.
Cabines allows up to 6 passengers and come in to flavors: open and closed. Open allows attacks at and from the passengers. Closed protects the passenger, but disables attacks. Cabines can be destroyed without killing the elepahnt, but killing the elepahnt destroys the cabine. Weigth rules which combinations of armour, weapons and cabines are possible.
Small ground dragon: low max weigth, low health, short ranged fire attack forward (ammo is sulfur, multiple shots per bit). You can buy saddle, ammo and body armour. Saddle and body armour similar to horse.
Ammo comes in different concentrations, higher concentration more damage.