Li Shenron
Legend
Why should he complicate things that much?
Just advance the horse by increasing its "animal class" HD: http://www.d20srd.org/srd/typesSubtypes.htm#animalType
It's the easiest way mechanically, and you know that the CR increases by +1 every 3 HD.
It's a progression, which means that the same individual horse can improve (you're free to say it's because of training or experience in battle), without the PC needing to retire a horse a buy a bigger one.
Just advance the horse by increasing its "animal class" HD: http://www.d20srd.org/srd/typesSubtypes.htm#animalType
It's the easiest way mechanically, and you know that the CR increases by +1 every 3 HD.
It's a progression, which means that the same individual horse can improve (you're free to say it's because of training or experience in battle), without the PC needing to retire a horse a buy a bigger one.