Theta-class raider docked at DS9
The Theta-class raider was a type of starship used by the Miradorn during the 2360s and 70s.
The Theta-class raider was armed with dual forward photon torpedo launchers and dual forward plasma charge emitters and could easily outgun a Starfleet runabout. Its maximum velocity was capable of exceeding warp 5. (DS9: "Vortex")
Background
The runabout's computer identified it as "Miradorn vessel, Theta class", suggesting this type of vessel is built by this civilization. Of course, it is unlikely that the Miradorn use the Human Greek alphabet, so "theta" must be a Starfleet designation.
Baran's raider, later seen in "Gambit, Part I & II" (TNG), was not specified as a Theta-class, however, based on its numerous character similiarities (maximum warp, similar ship and crew size), same generalized identification (a "mercenary ship"), and unmodified studio model, it may be reasonably assumed that is of the same (or similiar) class.
The Theta-class model was designed by Ricardo Delgado, and was subsequently re-used as the Taresian starship in "Favorite Son" (VOY).