The Mashur were a proposed army of advanced Humans which were going to be created by the Goa'uld System Lord Anubis to do battle with the Ascended Beings on their own plane of existence. Named after the Goa'uld word for mutation—mashur-nat—only the prototype being for the army was ever created by Anubis; Khalek. Had more beings been created, they were to have been trained by Anubis in order to ascend so that they could destroy the ascended Ancients. (SG1: "Prototype")
Behind the scenes
The Mashur were originally intended to be mentioned in the Stargate SG-1 episode "Prototype" but were cut from the final script, and are only known due to production notes leaked to GateWorld. However, as there is no contradicting information in canon about Khalek's purpose, it is entirely possible that the Mashur are still fully canon, though this is unconfirmed.