The Xanarian flea is presumably a type of insect.

Captain Rachel Garrett noted, following the death of their agent Laura Burke, that Starfleet Intelligence would be on Mahfouz Qadir, the head of the Asfar Qatala, "like Xanarian fleas." (TLE novel: Well of Souls)

It is unclear whether these insects come from a world called "Xanar," or whether their name derives from another source.