Dead Eye Plankton is a plankton (hence his name), that is short and green sharing an exact resemblance to Plankton featured on the show Spongebob Squarepants. He is about 2 inches tall with one uni-eye and a mustache with a top hat and a whip.
He is voiced by Doug Lawrence, who also voiced his descendant.
Besides the fact that Deadeye is wearing western clothes. Deadeye was one of the many ancestors of Plankton. He was featured in the episode "Pest of the West". He is a bad cowboy (as you can say) in 1800's trying to take over the town of Dead Eye Gulch (which was now named Bikini Bottom).
Dead eye's goal was to own the town and run Krabs out of business like his 200 years younger relative, Plankton, did. So he first started by invading the Salon and running Krabs out of business. Although he was only 2 inches tall, everyone was still afraid of him.
He forced the whole town to then be under his control and pay him very unreasonable taxes including personal belongings. Soon afterward Spongebuck came back into town after he got kicked out and accidentally challenged a duel with Deadeye at 12:00 pm.
So Deadeye and Spongebuck had their duel but Spongebuck accidentally stepped on the dead eye and then everyone noticed that he isn't as powerful as he seems and stepped on him and threw him in jail.