Well I have this problem, when you own a chocobo in my game your key name is assigned to a variable:

usr.yellow = usr.key

But that would mean that when you go to ride the chocobo and it checks to see if your key name matches the yellow var. then if you have ever owned one and your key name is equal to the yellow var. then you can get on it. I don't know how you would assign your key name to a specific mob say...

/mob/NPC/Chocobo_

So that when it checks to see if you own it, it checks the Chocobo and sees if a Variable of its has your name on it.(or any other way it would work.)