The Mathenites are an alien species.
In 2371, the Mathenite government gave Tekeny Ghemor political sanctuary after he left Cardassia, having been exposed as a member of the Cardassian dissident movement. (DS9 episode: "Second Skin")
In 2373, Erek Rhemet was rumored to have sought refuge with the Mathenites when Cardassia joined the Dominion and Skrain Dukat ordered violent purges of those in power in the previous government. (DS9 novel: The Never-Ending Sacrifice)
In 2376, a Mathenite woman was sitting next to Morn in Quark's. Morn appeared to be quite infatuated with her. (DS9 novel: Twilight)