Er...that's not really how it works. Hydras don't get Heads=HD because 'Hydra' isn't a monster, but a monster group. Each type of hydra has a different number of heads and is a separate monster. A 5-headed Hydra can't advance its HD, if you look at its MM entry. It can only be changed to a Six-headed hydra, or a 7+ headed hydra.<br/><br/>Now, they do scale evenly, so Zaer might allow you to be a 45-headed hydras your base monster, but gaining extra heads isn't a property of hydras by themself.