A typical adult Hydra, with one head regenerating.
Hydra are alchemic beasts, and are one of the common creations of the Arachne. They vary in size, but are usually intended to grow indefinitely. They have a pair of powerful front limbs, a long tail, and at least 3 heads, with no apparent upper limit, especcially as the Hydra get larger. Their coloration and integument can vary wildly, but typically take from the snake who's genetic material was used to create the beast, and thus can be quite regional. On their back are two large mobile sails, used to make the animal look larger, with exposed spine tips loaded with venom. All hydra have extremely potent venom in their spines and fangs, occasionally being flammable like in dragons and their relatives. Being created, Hydra are not true dragons or particularly closely related to them, but are occasionally still called Dragons colloquially.
Like many Alchemic beasts, Hydra are creatures of poor biology. They frequently have anatomical quirks such as kinked spines, extra limbs/toes, misshapen limbs or jaws, etc, and their heads don't get along well. They're known for their wild aggression caused by their constant pain and discomfort, and powerful, swift regeneration of body parts, which is powered by a lump of a magical material calls Hydrite. It only takes a week for a head to grow back, with more than one often sprouting from the same wound (but not always). Hydra will attack and mutilate new heads with significant deformaties with little consequence, until a "healthy" one grows. The Hydrite deposit is located in the chest, and can be removed without killing the Hydra (with great difficulty), which will just mean it can no longer regenerate, and will eventually die of its own infighting or biological breaking down.
Hydra are all-female, similar to their Arachne creators, and are self-fertilising. They lay massive clutches of eggs, with most dying or hatching with health problems. Their young look identical to common snakes in the area based on what their genes were derived from, with the exception of two small unusuable arms, which alongside the Hydrite will grow in and become usable by adulthood.
