Origin:   

Hydragons originate from the farthest eastern Russia, an ocean part called Bering Ocean. Very north of Japan. The Bering ocean connects the edge of Russia with the edge of Alaska in the very north. Some have migrated across Alaska as far Newfoundland. 

Over the long time period of migration, all Sea-serpents nowadays found have settled in large lakes of the northern regions of Alaska. They have adapted from salt-water habitats to the fresh-water lakes and even some rivers. This animal has become very rare in modern times.

Distinguishing Features:

The Hydragon is related to the common sea-serpent. It's greatest diffrence is, that it's body splits into 3 - 5 longs necks with a head on each of them. Hydragons can grow to a proud length of about 15 - 20 ft long. Hydragon is truly a bizarre creature, which basically looks like a gigantic snake with three heads. Like most snakes, the heads are small in comparison to its body. The body where all the necks are joint at their base is about the thickness of Hagrid's waist (belly width included). The body then tapers quickly into the tail.

Unlike snakes, however, the Hydragon has a sharp spiky crest running the length of each of its 3-5 necks. These jagged crests, which disappear at the end of the necks, are usually lying flat along the animal’s necks unless it is aroused by danger or the approach of food. The Hydragon is capable of dislocating its lower jaws, like most snakes, to swallow extremely large pieces of food. It also has a snakelike forked tongue. The long sharp fangs are hollow through the center and a narcotic poison can be injected into the Hydragon’s prey via these teeth. Thus a bite, though not lethal in and of itself, will result in the prey’s becoming groggy, stunned and disoriented, unable to escape, and the monster can finish it off at leisure.

One interesting feature is the “beak fang”. The front of the muzzle leads  to a sharp hard point. Though not an actual tooth, this sharp projection can be used by the creature as effectively as its teeth. Another trick of the Hydragon is to lash out at prey (or an enemy) with one of its heads, presumably to knock it over or into the water. Often this beak fang can inflict deep gashes with such blows as its very sharp.. 

Just behind each of its heads, the Hydragon has a set of gills, allowing it to breathe underwater. It can also survive for some time on land, so even if its prey gets loose, it can be pursued a short distance. This is unusual though, as the Hydragon does not like to leave the water and will only do so if extremely hungry.

Scales cover the upper part of the animal’s heads, necks, and body. A softer, leathery skin covers the belly and throats. This is the most vulnerable part of the Hydragon’s body, as the scales are almost dragonical hard. Most Hydragons are green in colour, though the shades vary widely, from a green so dark it is nearly black. There is no obvious external difference between the genders.

Nature:

Hydragons are not particularly vicious, usually killing only to eat, or if frightened or provoked. Their reputation for savagery has arisen from the fact that once they attack, they fight ferociously and to the death. A Hydragon, no matter how sorely wounded, will not stop fighting until it or its enemy is dead. The only exception to this rule is if the enemy has managed to retreat too far inland for them to survive. In general, however, they are more likely to be found playing in the water, diving and twirling around.  They hibernate during the winter, and are most dangerous in early spring, when they wake up ravenous and will eat anything they can get.

They pair up together in the summer and mate, after which the female digs a hole in the bottom of the Lake or river and lays a cluster of about 5 eggs. The parents do not stay with the eggs or pay any further attention to them, but go on their way. The eggs hatch in late summer and the baby hydragons, or hydragles as they are then called, are able to swim and hunt almost immediately thereafter. They are quite small, only about three handspans long, but grow remarkably fast. Within two years they are full adult size and ready to mate. 

Food: 

Like dragons, the Hydragon is carnivorous, eating only meat. They like fish of all kinds: Fish,  Crabs, Pirrangles, northern Balloon-frogs, etc. But the Hydragon hunts land animals too. It will go after anything that comes down to the water, from animals as small as a Kneazle to huge types of horses . Wizards are no safer, when the Hydragon is hungry.

Special magical feature and use:

The Hydragon's magical feature is, that if you cut off one of its heads it will eventually grow back from the stump. This adds to the horror most people have for these beasts, although according to the researchers, it seems to grow back only once.  However, the animal is capable of survival with only one of the 3- 5 heads remaining, though it would be severely handicapped.