Hydra (ヒュドラー, Hyudorā?) are serpents with nine heads in Greek mythology.
Hydra breathe a potent poison, and their entire bodies are filled with it. Simply standing near an adult one will make a human's lungs begin to rot. Even a deceased juvenile is great enough to harm someone with direct contact, and though not enough to kill a Servant, could still prove harmful to one. Being poisoned was not enough to kill a Divine Spirit like Chiron, but the constant pain was enough to make him give up his Divinity and immortality to cease his torment.
The Lernaean Hydra was battled by Heracles. It was an immortal hydra which would grow back its heads no matter how many times they were cut down. He utilized Nine Lives to simultaneously annihilate one hundred heads with a bow in order to slay it.
Rocco Belfaban owns a juvenile Hydra preserved in formalin, a small snake with its head split in nine. It is a one-of-a-kind item in the entire world according to Kairi Sisigou, and even one-third of its selling price would surpass his house's combined estate. He is able to process it into a potent poison for use in the Great Holy Grail War.
- ↑ 1.0 1.1
 Fate/Apocrypha material - Encyclopedia: Hydra [Magical Beast], p.181 [T]
Hydra [Monstrous Beast]
Among Heracles’ Twelve Labors, the monster he exterminated at Lake Lerna was the Hydra, a giant swamp snake said to possess either nine or a hundred heads.
Possessing both a deadly poison said to instantly kill all humans it touches and an abnormally high regeneration capable of immediately regenerating two new heads every time one is cut off, the Hydra is a Monstrous Beast with top class fame even among the other Twelve Labors. Shishigou Kairi accepted a young-born Hydra as prepayment and processed it. At first, he processed it into a knife as a final weapon that could kill even a Servant, but later he used it to make an antidote serum out of wariness towards Semiramis.
Though Mordred blocked herself out from it with her armor, it still wasn’t enough to completely block the air itself which was contaminated with the poison. The rank of the Hydra’s poison is undoubtedly top A rank. It might even reach A+ rank against heroes of Greek myth.