Alternate-Universe Squidward
Alternate-Universe Squidward is a creature whom looks exactly like Squidward Tentacles, except he is confined within mirrors. He is only seen in Squidward in Clarinetland. It is unknown what universe he is in, although it is likely that he is in the same universe as Alternate-Universe SpongeBob and Alternate-Universe Patrick. Alternate-Universe Squidward doesn't seem to like Squidward. It is unknown is he was an illusion, or a reality.
Another alternate-universe version of Squidward (possibly the same one) appeared in The Algae's Always Greener - with a much larger role. He is not in the mirror, but instead the Squidward in the "alternative-universe" that Plankton visits.