The Grand Serpent's species

The species of the Grand Serpent was a symbiotic species, composed of a humanoid and a spiked serpent-like creature, which could be psychically manifested outside of their humanoid form. The serpent-like creature could even manifest within an intended target, and if needed, kill the target upon exit of their body. (TV: Survivors of the Flux)

The Thirteenth Doctor identified this species as a binary demi-species, a very rare form of species in the universe. (TV: The Vanquishers)

Members of this species were accepted as a part of society on Inston-Vee Vinder's home planet, although it is unknown whether or not the planet is the Grand Serpent's original home or merely another of his conquests. (TV: Once, Upon Time)