Actually the hydra was the first monster made in the game (I like hydras). It was to be the Pythian monster. Every nation (about 5 at the time) was supposed to have one. Arcoscephale has the elephant. Then it ended
Monsters became summonables, but the hydra remained a recruitable for unknown reasions.
Sauromatian backwardly inherited the hydra from Pythium when it was made the predecessor of Pythium.