Let's have bosses that we can defeat and it would have 100% drop rate for the mount, but item that summons the mount will turn into ashes in 7 days. (very Ashes of Creation themed)
People defeat the boss and then it will drop one item only for summoning it's mount:
- the item will disapear in 7 days counting from when the boss was defeated
- the item is tradable
- the item has 100% drop rate, if the player gets killed the item will drop
- the server will have only 1 item of this or 1 boss of that item, never will have both at the same time
- after the item disapears then the boss will spawn in a random location
- can't be green if the item is in your inventory
- very few specific bosses could drop this and they would despawn and only spawn again after the item becomes ash
Such mounts should be way better than any mount, if it's a combat mount it should be a weapon of mass destruction.
The item can be held by one player only, only one of this item will exist at a time.
So guilds will have interest in finding where this boss spawned, they can sell, borrow, trade, pvp, they can do anything they please for one week... then it's gone, the item becomes ashes.
Other people will try to kill whoever has the item, if the player doesn't want to die with it, then the player will have to store it or trade it or wait 7 days to pass and the item will magically disapear from his storage.
Art from Ciruelo Cabral:

Only one person can have them at a time and it's better than having an attack helicopter.