A mercenary or soldier of fortune was an individual who offered to fight in exchange for money or other personal compensation. Species with a reputation as mercenaries included the Corvallens, (TNG: "Face of the Enemy") Nausicaans, the Breen, and the Klingons. (DS9: "The Magnificent Ferengi")
In 2369 Quark hired two Bajoran mercenaries for Rao Vantika to help him to steal a deuridium shipment from the Gamma Quadrant. (DS9: "The Passenger")
In 2370, the Vulcan Isolationist Movement hired several mercenaries, including Arctus Baran's group and Koral, to steal for them the remaining pieces of the Stone of Gol, an ancient Vulcan weapon. (TNG: "Gambit, Part I", "Gambit, Part II")
Also that year, Verad hired two Klingon mercenaries, T'Kar and Yeto, to help him take Jadzia Dax's symbiont by force. (DS9: "Invasive Procedures")
The Takrit in the Delta Quadrant are known for being mercenaries. In 2372, the Trabe hired the Takrit to reconnoiter the meeting place of a proposed summit between Kazon majes, in preparation for an assassination attempt. (VOY: "Alliances")
In 2374, during the Dominion occupation of Deep Space 9, Quark intended to hire Nausicaan mercenaries to break his brother Rom out of captivity. Kira Nerys however, convinced him that an attack would cause unacceptable casualties. (DS9: "Favor the Bold")
Later that year, Rom wanted to hire mercenaries to rescue Ishka from the Dominion, but Quark insisted on an all-Ferengi team instead. (DS9: "The Magnificent Ferengi")
The only game of tongo Miles O'Brien had ever played was against a Ferengi privateer and a Romulan mercenary. (DS9: "Change of Heart")