Every hydra has grown from the severed hydra head of a larger hydra. Severed hydra heads, no matter the size, are able to grow appendages and escape the attacker. After some time, they transform in the the body of a hydra.
Lernaean Hydra: The Original. Immortal, nearly impossible to kill or survive an encounter. Has telepathic control over all hydrae. Many poison-related attacks and powers.
Greater Hydra: Very dangerous. Appropriate for characters level 10+. Greatly resent whomever severed it from the Lernaean Hydra. Often holds many Lesser Hydrae in its control.
Lesser Hydra: Appropriate for characters level 8-10. Cannot remember Lernaean Hydra, seeks to escape domination of its progenitor Greater Hydra.
Small Hydra: Suitable for characters levels 3 and under. About the size of a dog. Lives as a wild animal.
Micro Hydra: Small creature with a deadly bite. Captured and used by assassins. Easily crushed.
What I hope this offers you as a DM is the chance to use the monster at almost any level of play without taking away the awesomeness of the original Hydra of myth. Hydrae should be one of the truly frustrating and deadly monsters in any D&D campaign, and this series gives hydrae dominion over poison, a position they held in the myths of old.
Please let me know if you decide to give them a try. I have in most cases left the final HD of the monster in the hands of the DM, who will decide how tough the monster is by the number of heads it has. If there’s some tweaking needed, I’d love to know.