T-800s, after Terminator, are advanced zombies that were introduced in Zombieland: Double Tap. "T-800" was the name that Columbus gave to them, whereas Flagstaff referred to these zombies as Bolts for their speed, after Usain Bolt.
They are faster than normal zombies, but their main distinction is their seeming indestructibility. The first time Tallahassee encountered a T-800, he "killed" it via the usual double tap method, only for it to get back up again. Tallahassee repeatedly shot in the head until he ran out of bullets, and although the T-800 dropped to the ground, it continued to crawl toward Tallahassee until he crushed its skull with his foot.
In the second T-800 encounter, they had followed Albuquerque and Flagstaff to Nevada's motel. Although the pair defeated the T-800s, they were bitten, and turned into T-800s themselves and at a far more rapid rate than regular zombies in Zombieland. This indicates a more virulent infection, and that the strain of virus dictates the form of zombie a person takes.
A massive super herd of T-800s attacked Babylon. Tallahassee discovered them when he hit a T-800 with his truck, breaking its neck, but it stood up, fixed its neck and took off towards Babylon with the rest of the herd. Some of the herd was killed with exploding bio-diesel and run over by a monster truck, but most died when Tallahasse lured them off of the roof of the Babylon skyscraper.