Hydra are fascinating creatures. Born from eggs about the size of a human head, a hatchling hydra resembles a two-headed python roughly a yard long. Once it finds prey, the creature begins to grow at a truly impressive rate. If food is plentiful, a hydra can reach maturity within a year. During its growth process, the young hydra will often chew off and eat one of its own heads, triggering the regenerative process for which it is best known, as two new heads replace the one.
The key to the hydra's truly impressive resilience lies in redundancy. An adult hydra with seven heads has three separate lung clusters, a dozen two-chambered hearts, nine brains, and four stomachs leading into two parallel sets of intestines. Each lung cluster is connected to a minimum of two and a maximum of three heads, with valves allowing the creature to inhale through one head and exhale from another. Each head and neck has its own dedicated heart, with additional hearts feeding the body and internal organs. During a battle, as the hydra gains or loses heads, its internal organs will shift, form, or dissolve, in order to best accommodate its needs.
Unlike most vertebrate creatures, the hydra's spine lies near the core of its body, surrounded by its various organs. Ribs branch away from the spine in multiple directions, forming a protective ring around the spine and organs, and functioning as muscle anchors for articulation. A typical adult hydra keeps a host of small sacs along the length of its body, outside of its ribcage. These provide padding behind its tough scales, and contain stores of spare body mass, which the hydra can channel to any part of its body, fueling its impressive healing.
Each head contains its own brain, which interprets sensory input and controls that head and neck. A brain nested inside the spine a few feet behind the division point of its necks acts as a coordinator, and another motor brain further back along its spine controls its tail. Each motor brain seems to process sensory information for its own segment of the body. Motor brains in the heads act upon simple concepts of Food, Threat, or Friend, but recognition of individuals or commands as one might associate with a trained animal depends on the coordinator. We know this in part because the hydra's heads do not display independent personalities, like one might observe in an ettin or other multi-headed creature.
Some rarer specimens of hydra possess an additional talent. Their heads grow fluid sacs in the corners of their jaws, similar to a poisonous serpent. Unlike a serpent, however, the hydra's fluid sacs contain substances that react violently when exposed to air, either combusting or freezing as the hydra sprays these substances from channels in the roof of its mouth. Creatures with this ability demonstrate extreme resistance to the effects of fire or cold, similar to that seen in dragons. Because of the additional hazard such specimens present, attempts to domesticate one have thus far proven untenable.