Small bat-like creatures, one of which lives among the medicinal animals in Phlox's sickbay aboard Enterprise and escapes its cage in A Night in Sickbay.
The Pyrithian bat is a member of the menagerie that Dr Phlox keeps aboard Enterprise NX-01, a collection of live animals he uses as sources of medicines or as food for other specimens. The bat lives in a cage in sickbay, and Phlox is evidently fond of it.
It is best remembered from the episode A Night in Sickbay. Archer, sleeping in sickbay to be near his sick dog Porthos, is drawn into a late-night chase when the bat gets loose, and he and the doctor stalk it round the room with nets. The animal is eventually recaptured, one of several interruptions to the captain's rest that night.
The animal is seen or mentioned in other episodes as part of the routine of sickbay, including occasions when someone else has to look after the doctor's creatures in his absence.