A Pyrithian bat was a flying mammal with white fur and primarily dieted on snow beetles and moth larvae, only eating live food. The bat's natural predator was the Pyrithian moon hawk. Doctor Phlox kept a pet female Pyrithian bat among his many animals aboard Enterprise NX-01.
In 2152, while Captain Archer spent a night in sickbay, the doctor's bat got out. The two spent part of the night trying to capture it, and Phlox made a paper Pyrithian moon hawk and tried to impersonate the call of it to scare the bat. It was less than successful, but Hoshi Sato was able to catch it relatively simply. (ENT: "A Night in Sickbay")
In the Delphic Expanse, Phlox offered Malcolm Reed some of his bat's moth larvae to eat, after Reed had transformed back from being Loque'eque, and still found his stomach was sensitive to Human food. (ENT: "Extinction") Phlox later used his bat as a distraction while he dispatched a Triannon guard in November of 2153. (ENT: "Chosen Realm") He claimed that there was no antidote for its venom, though it was not actually venomous. (ENT: "Chosen Realm", "Dear Doctor") He told the bat not to bite his fingers again while feeding it a moth larvae, just before he ate one himself. (ENT: "Chosen Realm")
Origins of name
The use of the term "Pyrithian" was originally introduced in Star Trek: Voyager in association with the Pyrithian Gorge, located on the Talaxian homeworld, in the Delta Quadrant. Since the Pyrithian bat and Pyrithian moon hawk, two species known to the Denobulans, were presumably native to a planet in the Alpha or Beta Quadrant, it would seem apparent that this term was not related, and was simply used by two distinct cultures, such as the term "Darmok".
In the writers' first draft script of "Dear Doctor", the Pyrithian bat that Phlox kept as a pet was a male rather than a female. When Hoshi Sato arrived in his sickbay (at the start of the episode's first act), Phlox told her that she had just missed her chance to feed the creature (whose species name was misspelled "Pirythian bat"). Sato readily accepted this news, telling Phlox that the last time she had fed the animal, it had become "a little jumpy" around her. However, Phlox reminded her the animal only bit males. Nonetheless, Sato was skeptical the creature could tell the difference between the genders of whoever fed it.
In the final draft script of "A Night in Sickbay", the Pyrithian bat was described as "large" and "dark blue."
In ultimately omitted dialogue from the final draft script of "Damage", Phlox mentioned to T'Pol that his Pyrithian bat was in heat and that, as a result, it very often made a "cooing" noise that Phlox described as "incessant" and "trying."
According to Star Trek: Star Charts (p. 44) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), there was a star system named Pyrithia (26 Draconis), which might possibly be the system of origin for this species, in the Alpha Quadrant. This was a trinary system with a G-class star, an M-class star, and a K-class star. This system was a destination on a Denobulan trade route in the 22nd century.