Beast Factor

Beast Factor (獣因子) is a genetic component that allows beastmen to transform into their beastman forms in BNA: Brand New Animal.

Description
In addition of endowing a beastmen their animal form, Beast Factor also endows them with superhuman abilities related to their animal form in question. For example, bird beastmen possess wings and talons for flight and grabbing things respectively like birds and rhino beastmen possess enhanced strength and durability in addition of horn to damage enemies with.

The extent of a beastman's shapeshifting ability seems to relative with how they possess this genetic component. Whereas regular beastmen mostly limited to a single anthropomorphic animal form (with notable exception of Shirou who exposed to souls and blood of his fellow beastmen inhabitants of Nirvasyl that resulted his "transcendence" into the original Ginrou), those who received beast factor through artificial means like Nazuna Hiwatashi and Michiru Kagemori can freely assume any animal form they wish, including a full animal form. As beast factor's condition is relative to the bearer's emotions, he/she must not succumb to stress to prevent it from consuming them, a condition that referred as Nirvasyl Syndrome.

Biological or Supernatural?
Although the series established beastmen as biological species, unique powers displayed by Shirou, Nazuna, Alan, and Michiru showed that beast factor possesses supernatural elements, a fact purebred beastmen are aware of. According to Alan, a beastman may transcend into a god-like entity by being ritualistically bathed in blood gathered from a thousand beastmen of the same species (in his case, wolf), though accidental exposure will do the trick even if the blood in question being taken from multiple beastmen species at the same time. The latter method of transcendence however, had a side effect where the recipient's body will house souls of the owners of the utilized blood via. beast factor within. Whereas such downside explicitly viewed as an affront by purebred beastmen through Alan's expression during their final battle, Shirou argued it as a gift since among souls inside him are in fact, his friends and loved ones who perished during Nirvasyl massacre, encouraging him not to give up in his campaign to save his fellow beastmen and thus lived up expectations from being the legendary Ginrou and a protector. In the case of human-turned beastmen who received their beast factor through a botched blood transfusion on the other hand, they displayed abilities that not associated with any known species, such as extending and enlarging one's limbs at will, generating blinding light through excess energy, mimicking virtually every single one of beastmen's known powers including those of transcendent ones, and other unknown abilities. Thus, it's plausible to assume that beastmen were the result of evolution controlled by an unseen higher beings.

It is possible the primal beastmen could pull off similar feats. According to the mayor, the original beastmen apparently obtained their animal forms from on the animalistic gods they worshiped, presumably out of a psychological desire to be closer to these gods, and thus they had no set form beforehand and might have just been shapeshifters though this is just conjecture at this time.