The Chaos Serpent is a creature encountered in both Heretic and Hexen. There are three versions of this beast:
D'Sparil's Steed - Appears to be the strongest of all Chaos Serpents, can shoot 1 or 3 fireballs at once depending on health. Also can run with a small burst of speed when injured. Has 2000 Hit Points.
Green Chaos Serpent - Appears identical to above, but without a rider. Has 250 Hit points and can breath single shots of fire. Can also bite at close range.
Brown Chaos Serpent - Brown skinned Chaos Serpent. Has 250 Hit Points and fires single balls of poisonous smoke at enemies. Can also bite at close range.