The hydra is a many-headed dragon that lurks in swamps. Simple and bestial, it is known to prey on anything in its home terrain. Famous for its regenerative powers, some even consider it immortal. As they grow older and more powerful, they grow additional heads, becoming much more deadly in the process.
Hydras attack with their multiple heads, biting in a flurry of attacks that few can defend against. As they take damage, the heads are lost, until the beast is finally slain, the last head chopped off.
A breed of hydra possess remarkable regenerative powers, growing more heads as they take more damage: for every head chopped off, two grow in its place, enhancing the damage dramatically. The regeneration can be halted with fire, burning the stump, and preventing more heads from growing.