(covers information from several alternate timelines)
The Menk were one of two native humanoid species on the planet Valakis. They shared the planet with the more advanced Valakians. The Menk were far less evolved than the Valakians, who treated them as the primitive species they were. However, the Valakians provided the Menk with food, clothing, and other needs.
Humanity made first contact with the Menk in 2151, when Enterprise NX-01 picked up a Valakian shuttle and returned them to their homeworld, where it was discovered that a genetic disorder was slowly wiping the Valakians out. When Dr. Phlox was asked to help, he had a hunch the answer might be in the Menk's immunity to it. His thoughts were correct, but they also found that the Menk were on the verge of an evolutionary breakthrough, showing signs of increased intelligence, which might lead to them becoming the dominant species on the planet.
While several Ferengi later raided Enterprise, one named Muk found a device in the starship's sickbay and, when told to add it to the Ferengi group's collection of stolen goods, he exclaimed, "Do I look like a Menk to you?" before insisting that he keep the device for himself. (ENT: "Acquisition")
In February 2259 of the alternate reality, Starfleet tracked the movement of Menk representatives in the San Francisco Metropolitan Area. Their current position was ID'ed by a label superimposed over a map of the area. This map was in a collection of graphics and video media that was seen on a powerwall in the office of Admiral Christopher Pike at Starfleet Headquarters. (Star Trek Into Darkness)