Premise
For as long as humans have recorded their history, there has always been mention of those that could change form. The Egyptians, for instance, had worshipped strange half human, half animal 'gods'. While the Native Americans believed that people had spirit animals that guided them and reflected their true character. Mythology features many creatures that can take on or show human characteristics. And then there are the stories of dark and horrifying were-beings that cursed people turn into once every full moon. All these stories have on thing in common - their true basis is about Therians.
Therians are not cursed people; like the were-beings of the horror stories; nor are they half one thing and half another. They are normal human beings like everyone else. Except with one fundamental difference. They have the ability to change into a specific animalia form. This ability is not brought on by any outside influence; not by being bitten or scratched by another similar animal form; but is in fact part of these select humans' genetic make-up. Some individuals will know of their Therian gene due to their other family members; while others may have no immediate family trace of it.
In modern times, the Therians keep their ability secret and live amongst the rest of humanity. Although, there is a specific town where Therians thrive. This town was established many years ago by one such Therian who wanted to have a place where they needn't fear the other inhabitants nor keep their secret. Although, in recent times, many non-therians have moved to live there and thus the abilities are kept from those that do not know of their existence.
Although the Therians are not cursed like the were-beings of the stories; nor do they particularly like being referred to as weres; and can change at will - or if the 'flight or fight' instinct kicks in - there is a specific time when their change is forced and out of their own control. An event that happens once a month and is responsible for the myths of werewolves and the like. The full moon.