[OmikronWarrior]

Basically numerous Dom3 units have "multiple shapes." One example would be Midgard's skinshifters. The HP listed is actually only for the first form. When this form runs out of HP, instead of dying it changes into the next form with a new HP total. This can be interpretted different ways for different units. For the Hydra, the HP listed is for a single head only, and numerous such "heads" have to be killed to get the "real" hydra form, which will have a much higher HP total.