Gort

Gort is an alien robot made out of nanites.

History
When Homer saw an alien, FBI agents Mulder and Scully came to Springfield to investigate his sighting. They assembled a lineup of aliens which included Marvin the Martian, Gort, Chewbacca, ALF and a Rigellian (either Kang or Kodos), and asked Homer to identify which one he had seen. However, it wasn't any of them. Homer's alien was eventually revealed to have been Mr. Burns, who had picked up a habit of wandering into the woods following his weekly rejuvenation treatment sessions.

Gort was later seen an the Bi-Mon-Sci-Fi-Con signing autographs.

Non-canon
Gort was seen when the camera panned through Springfield.

Gort was working as a waiter in The Gilded Truffle.

Behind the Laughter

 * is a robot from the 1951 film , which was in 2008.
 * Gort's appearance in the lineup is based on the original film. The episode far predates the remake, and in the remake Gort is much larger.