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Different take on the Hydra.
Any combination of Water, Dark, Dragon, or Poison typing.
Ability: Schooling or something like it
With schooling it would start as a simple serpentine creature and, at a certain level, call a swarm of serpents to attack the enemy.
With one similar to schooling it could function like that Zygarde ability. When it hits 50% or less health it triggers a transformation that summons more and more of the creature to create a giant, many-headed beast.
Both could convey the "cut off one head, two sprout up in its place" saying associated with it.