The Green Hydra is a Legendary pet in Bubble Gum Simulator. It can be obtained by opening a Nightmare Egg. The chance of hatching it is 0.01% (1 in 10,000) or 0.02% (1 in 5,000) with the Lucky Chancesgamepass.
It could also have been obtained by purchasing the 94th slot at Christmas Island. It can now also be obtained by blowing 200,000,000 bubbles and claiming the prize.