An Unknown Alien race that seems to inhabit the Pegasus galaxy in an alternate reality was seen by Lt. Colonel John Sheppard's team when they were transported to an alternate reality by the Alternate reality drive found on an alternate Daedalus which appeared over M35-117.
The unknown aliens seem to be in conflict with Earth and the Atlantis expedition. It is unknown where they originate from and if they are at war with the Wraith, or how they have concealed themselves from them to progress to the technological level they are at. (It is possible the Wraith do not exist in this universe. It is also possible that they are the Wraith, albeit along a different technological path.) They possess powerful ships that rival those of the Tau'ri. They do however share some resemblances to the wraith, most notably their teeth and facial structure, but they do not use biological ships or technology. They seem to be physically strong, as one was capable of defeating Ronon Dex in hand-to-hand combat. They also seem to be very resilient to weapons fire, as it takes several rounds from a FN P90 Personal Defense Weapon and Ronon's Particle magnum to take one down. They have some sort of synthetic implant in the left side of their skull that can be seen flashing red, and they seem to have a weakness to bright lights.
This unknown species was encountered in an alternate reality, where they were attacking Atlantis. The alternate Daedalus was able to disable their ship before they jumped to the next reality. However, at least four of those aliens boarded the ship after one of their Fighters crashed with the Daedalus, those encountered were eventually killed.
The Daedalus later returned to the same reality and encountered the ship again, still disabled. The Daedalus had suffered heavy damage from fighting the ship's fighters but was later saved by the Atlantis expedition of that universe who helped destroy the fighters in F-302 fighter-interceptors. It is unknown what happened to the battleship after the Daedalus jumped away but it is presumed it was destroyed by the F-302s. (SGA: "The Daedalus Variations")
In an alternate reality, three months after the defeat of the Super-hive above Earth, these aliens attacked MGX-482. The aliens swarmed through the gate overwhelming the security personnel. After driving the survivors into hiding, the aliens began to establish a beachhead using the Stargate to walk back and forth through the event horizon, a feat that Radek Zelenka believed was impossible, and surmised that they were invading between realities. Colonel Steven Caldwell managed to beam the survivors out from the Daedalus. The aliens eventually abandoned the planet on their own accord. However the Ladon Radim of the Genii soon later arrived to plead for the aid from the Atlantis expedition as the same aliens had conquered the Genii homeworld. (SGA: "Back to Pegasus 1")
The Daedalus investigated the Genii homeworld and discovered that the aliens were building an armada. Guide provided the Atlantis expedition with more background on their origins. The aliens were more than a legend amongst the Wraith and were regarded as formidable foes. During that time several hives en-route to a culling encountered an derelict alien vessel. Unfortunately the ship was not abandoned and the aliens attacked and boarded a hive, nearly overwhelming the Wraith. The Wraith made the decision to ram the alien vessel destroying both ships. The remaining hive ship was able to dispatch the aliens and discovered two critical pieces of information. One being that the Wraith were unable to feed on the aliens, and that the aliens were of Lantean design. On New Athos, the aliens had ravaged the planet and reports on multiple worlds were that local populations were being enslaved. (SGA: "Back to Pegasus 2", "Back to Pegasus 3")
The Atlantis expedition then sent AR-1 to the Tower Planet to salvage a control crystal. The world was then attacked by the aliens, thought Sheppard managed to activate the Control chair and destroy the hostiles using the drones. Unfortunately the aliens came in greater numbers, forcing the humans to retreat into the Tower. Sheppard then decided to activate the buried city's shield, excavating it and causing multiple fissures that killed many of the aliens. (SGA: "Gateways 1", "Gateways 2")
Later, it was revealed that the rogue Ancient Janus had assumed control over the aliens. The aliens were a product of his creation, a weapon from an alternate version of him that created them to eliminate the Wraith. Though the alien's Janus was deceased, protocols within them forced them to obey this current Janus.. Janus gave them a directive to not to harm Colonel Sheppard as he wished to deal with the human personally. (SGA: "Gateways 3")