Multiheaded serpentine lake monster (5)

I believe the answer is:

hydra

(Other definitions for hydra that I've seen before include "that regenerated after cuts at the top" , "Nine-headed monster of Greek myth" , "Mythical many-headed snake" , "Persistent trouble" , "Many-headed foe of Hercules" .)

I've seen this clue in the USA Today.

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