Stragoth

These aliens are of unknown origin, however they were first encountered on P3X-118. They managed to infiltrate the SGC by using their mimic devices to impersonate SG-6, eventually taking over the facility. Their mimicking technology did not work on everyone; Teal'c because of his Symbiote and Samantha Carter because of her altered body chemistry from being a former host. While on an airplane, a noise produced from the jet engines disrupted the mimic devices, making the aliens visible for a short time. Carter was able to discover the specific frequency affecting the devices and broadcasted it throughout the SGC. When Colonel Maybourne's forces pentrated the facility, the aliens attempted to leave through the Stargate. After several escaped, Carter was able to shut it down. With no other way out, the aliens self-destructed rather than be captured.