Read the story of Hercules and the Lernean Hydra, as told by Apollodorus, the ancient writer who collected legends in his mythology handbook, the Library.
The Temple of Zeus at Olympia was decorated with sculptures illustrating the Labors of Hercules. Look at the sculpture catalog entry for the Lernean Hydra metope.
Hercules' arrows, poisoned with the hydra's venom, were deadly to the centaurs he met during his fourth labor. Pausanias wrote that the hydra's venom polluted a Greek river, the Anigrus, after the centaurs washed their wounds in it.