Hydras originate from Greece, where they featured in ancient folklore as a result of their occasional attacks on trading and fishing vessels. Hydras are aggressive aquatic serpents that make their nests in coastal caves. They are very rare on account of being trounced by the numerous human heroes of that region, and also because various evil overlords and beastmasters are always capturing their young to be used as guardians for some dungeon or something. The forces of Evil rarely have decent breeding programs for their hapless slave-beasts.
In modern times, the hydra has had trouble adjusting due to its unique biological quirk of having several completely functional brains exercising equal control over its body. In the past, it was easy for the brains to unify their thought processes, because the typical hydra only had one: kill every living thing and eat it. In today's society where information is widespread and constantly catering to one's animal urges is unnecessary, this has led to an unfortunate phenomenon in which different heads will develop distinctly different personalities, with wants and needs different from, if not directly opposed to, those of the other heads. This can lead to the appearance of having multiple personalities when trying to pass as a human, and can even lead to personal infighting in severe cases.
Given that the more heads a modern hydra has, the harder it is to focus and accomplish anything, young hydras have it easiest; a baby hydra has only one head. They grow additional heads as they grow older, although this phenomenon takes longer to occur after each growth. It's typical for hydras to effectively stop growing new heads after the seventh one, though only very, very old ones get that many, and it's rumored that ones with nine and even ten heads exist.
In combat, hydras mostly rely on their bulk and numerous vectors of attack, as they find agile movements all but impossible and can't easily compose a plan of attack unless it has time to stop and debate a strategy with all of its other brains. Due to their body structure they're quite clumsy, and despite being an aquatic serpent, they're not much more agile in the water. They do have the ability to spit water with rock-pulverizing force though, and they can regenerate wounds fairly quickly! A hydra can easily exhaust cautious opponents with its considerable stamina and random attack patterns, while more aggressive enemies will find that it's hard to keep a hydra down when you have several equally important targets that keep shrugging off your blows. Once again, the solution to regeneration is fire