So, that's the hydra. (Spoilers)
Of all the Hercules movies of the 1950s and 1960s, this is the only one I know of to tackle what is arguably Hercules's most famous fight. It was an interesting idea to have Hercules get knocked out after fatally wounding the monster. If there's a monster that can knock out Hercules, the hydra is a good candidate for it. Too little, too late for him though.
It's also kind of interesting that Hercules kills the hydra by severing its middle head. According to the myth, two heads grow back for every one that's severed. But then this movie also has a fairly wimpy Achilles and an old man for Iolaus (Hercules's nephew in the myth). I suppose it goes without saying that this isn't exactly the most accurate interpretation of the story. lol.