Films: Hydra (2009)
Location: Cave/Forest/Civilized Area
Height/Weight: That of a bus.
Summary: One of Hercules' most recognized labors was also one of his first. Namely, kill the dreaded 8-headed Lernean hydra. Like every labor before and after, he did it with some quick thinking and lots of courage. But Hercules isn't here anymore, and the beast is back...
History: On an isolated Mediterranean island, three separate groups are at deadly odds with one another. There's a group of rich idiots looking for some killing sport. The next is a bunch of ex-convicts fighting to survive. And finally, there's a group of four...and they're all part of the same body. For you see, this is the island of the Lernean hydra, whose looking to snack on everyone.
Notable Kills: Any time this thing uses all of its increasingly numerous heads to tear people to shreds.
Final Fate: Eventually, someone manages to get his hands on the Sword of Hercules, and uses it to permanently cut off many of the hydra's heads. After it's brought down to two and invades the boat out of here, the sword comes down on it one last time. Once all heads are sliced off, the rest of the body dissipates.
Powers/Abilities: For every destroyed head, two take its place.
Weakness: Only the sword of Hercules stands a chance of cutting the heads off with no problem. If all heads are chopped off in time, the hydra will cease to exist.
Scariness Factor: 3.5-For such a poorly-rendered beastie with hot dog-like heads, this hydra perfectly encapsulates the terror of the original Greek myth. You need the right weapon to meaningfully harm it, and if it gets one of its jaws around you, you're going to see exactly how a hydra shares with itself. And as one might guess, it isn't pretty.
Trivia: -In the original story, Hercules is only able to destroy the hydra's heads by burning them off. In several interpretations, there is one immortal head, and Hercules has to bury it in the ground so that it doesn't rise again.
-There are real-life hydras, but they are nowhere near as large nor dangerous. And they aren't reptiles either. They are microscopic amoeba-like creatures that can regenerate, and may very well be immortal themselves.