In Greek mythology, a monster with nine heads, each of which was replaced by two new ones when cut off (5)
I believe the answer is:
hydra
(Other definitions for hydra that I've seen before include "Many-headed foe of Hercules" , "Water monster" , "Mythological water-monster" , "Many-headed snake slain by Hercules" , "that regenerated after cuts at the top" .)