Tail Monsters were created in a lab, in an attempt to augument humans for various purposes, such as for dangerous tasks or use as living weapons. The tails were created using a variety of genes and some strange objects found in the area. The lab is in a remote location, although it is rumored to still be operational and producing new Monsters. Occasionally Monsters escape into the surrounding area, or people and animals are exposed to the items necessary and become Monsters themselves, so it is not unheard of to see Monsters in the outside world. It is unknown exactly what the tails' capabilities are, but they seem to generally be able to reason and act reasonably independently from their hosts.
Tail Monsters generally look like humans, although animal-like Tail Monsters have also been found. As quite a few of them weren't created with the best methods (either because the lab wasn't aware, or due to being accidental), it isn't uncommon for humans and their tails to be unable to get along. In addition, the tail's presence often changes other parts of the host body to suit their combined survival, especially if it's large and needs better support. In addition, the human hosts that were given tails in the lab often have additional augmentation, whether cybernetic or otherwise.
A Tail Monster's tail is a lifelong companion, as it is attached to and merged with the spine. Tails can be almost any shape or size, but must have a mouth and usually have eyes. Tails can also have arms, sensory organs, or other appendages, though they can't have a full body. To make up for this, the same mechanisms that allow for the creation and binding of a sentient tail also allow for the editing of the host's body. If a tail is created with and bound to a strange object, it will be stable and have that object as its "core". If not or if the object is damaged or destroyed, it will become unstable. In animals that already have tails, the new tail will merge with and replace the original tail.
As the Tail Monster is stuck with its tail for life and they need each other to survive, the two often try to work together, although sometimes they just can't get along. Most tails are incapable of communication (including with their host) by means other than animalistic sounds, the type depending on their appearance and structure, but a few are able to communicate through other means (such as sign language or telepathy). Tails that were created towards the beginning of the lab's research or accidentally tend to get along with their hosts the worst. Some Monsters have muzzled tails so they don't try to attack them, but this means that the host has to feed both of them.
Tails can have any diet, and they often eat for their host as well. This, combined with the tail's creation changing the host body, means that the host can be forced to adhere to a specific diet. Larger tails can sometimes eat entire animals, which often requires significant modification of a human host.
For some reason, tails tend to be "born" with one or more special abilities, which may also be shared with the host or cause further modification. These include things like flight, stretching to eat large animals, or magical powers. It is hypothesized that since a strange object is necessary for the proper creation of a tail, that they can be accessed by the tail.
Strange objects are mostly necessary for the creation of a new tail, as they help keep the tail from becoming unstable. Strange objects usually take the form of crystals or contained liquids, but may vary. It is unknown particularly what they do, but it is known that they can stabilize tails. If a tail is created using a strange object, it will be inside of them, as a sort of core. It's nearly impossible for them to be damaged in this state, but it still happens, like breaking a bone. If a tail isn't created using a strange object, it will lack a core and be unstable. A strange object can be added, however, either by surgically implanting one in the host or by placing it at the base of the tail. An object implanted in a host will instead give the host abilities, as well as changing the appearance of the surrounding area. Placing one at the base of the tail will give both the host and the tail abilities, but it is easy to damage or destroy. For this reason, a protective shell or horn often grows over it.
When a tail isn't created using a strange object, or the strange object is damaged or destroyed, it becomes unstable. Unstable tails are aggressive and can drastically change if not fed strange objects (usually crystals or liquids). Since their host cannot usually derive nutrition from these objects, they often have significant physical changes to their appearances. Most unstable tails only eat strange objects, as they know it's necessary for both their and the hosts' lives. Unstable tails also tend to have either corrosive or mutagenic saliva, the latter of which helps to create new Monsters. In addition, their appearance often changes to be more similar to their object of choice, which can rarely make them stable again.
Tail Monsters can be bred if they have the same host species. The tails will also pass appearance and traits of the offspring, but strange objects are necessary to prevent them from becoming unstable. Monsters, regardless of host species, hatch from eggs. All tails are protective of Monster eggs, and will happily take care of any eggs they find until their tails are fully developed, even going so far as to find strange objects for the offspring and protecting them from harm.