wraithstrike |
If this is only a change in appearance and the mechanics dont change I would just go with a flavor based reason. As an example there have been golems with slams attacks that have had different shapes than the normal golem. Since it is just an artistic change no extra money is spent.
If the rabbit form makes the demon worse there should also be no cost. Just say he was cursed or something.
Basically, you dont need mechanical justification for everything. If fleshwarping was not in the books and an author wanted to do it they would just say it happened.
In addition there is no way to answer the "how much" because there is fleshwarping has no value attached to it that can described with an amount. If it used a point system like animated objects do that would be different.