The M'klexa were a warp-capable civilization who had visited Valakis as of 2150.

As of 2151, T'Pol was not familiar with this species. (ENT: "Dear Doctor")

This species was only mentioned in dialogue.
Prior to meeting the Humans, the Valakians were visited by two warp-capable species, the M'klexa and the Ferengi. Nothing is known about these encounters, but apparently neither species was able or willing to help the Valakians.