I think that when you do /purchase animal you should get a spawner egg, not a spawned animal. Animals that I have spawned have suffocated in walls and a spawner egg would just be easier. Also, if you use spawner eggs, the player would be able to spawn any non-hostile mobs. (Like baby slimes). Also, I think chickens should only be 25-50r, cows and sheep 250r, pigs 125r and mooshrooms 350r. Or something similar with chickens and pigs cheaper, and mooshrooms more expensive.