The Grand Serpent's species

The species of the Grand Serpent was a symbiotic species, composed of a humanoid and also a spiked serpentine creature, which could be psychically manifested outside of their humanoid form. The serpentine creature within could even manifest inside 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)

Behind the scenes
Some writers have noted numerous similarities between the Grand Serpent's species and the Mara, introduced in TV: Kinda.