[Edi]

Hydras have 6 hp per head, 100 regen for those heads. The main body has 40 hp and 30% (12 hp) regen. That's a total of 88 HP. That makes them immensely durable when used in groups. The weak poison from each hit stacks up every time it hits, so unless a hydra gets swarmed and annihilated immediately, its opponents are going to die of poison very quickly.

In one of my games, I have Fomorian giants with a N10 bless (20% regen = 13 hp per round) and my stack of 20 blessed Fomorians had their hands full dealing with a mere half dozen Sauromatian hydras and their support chaff. The reason being that they were taking nearly as much damage from poison every round as they were regenerating AND they were getting the damage from the blows as well.

The SC hydras, when fixed, are going to be begging for a F9W9 bless.

Oh, and Serpent Cult has a hydra special hero. In place of the Great Head weapon it has something called Immortal Head and the bugger is sacred. Draw your own conclusions...