Nagini was a female Maledictus (Maledictus is a female individual who carries a blood curse that eventually turns her into a beast permanently. The curse is carried from birth and passed down from mother to daughter. The beast ultimately transformed into can vary based on the curse).Originating possibly in Indonesia by 1927 she was a performer at the Circus Arcanus where she met Credence Barebone and had the ability then to transform at will. By 1994, she was seemingly trapped in snake form and belonged to Lord Voldemort, with whom she had a special bond largely due to becoming his Horcrux and him being a parsletongue after her master had killed Bertha Jonkins in 1994.

How Nagini Can Be a Human in Fantastic Beasts 2 - YouTube