The Jockey

The Jockey is a new form of Special Infected that will appear in Left 4 Dead 2. It will attack Survivors from behind and jump onto and cling to their head and upper back. From there, they can control the Survivor while doing so. It appears that the Jockey has been mutated by the infection to the extent that he has a hump, which is what causes him to hop around in a spider monkey like motion. However, he can jump great distances, yet nowhere near as far as the Hunter. His lips and the skin surrounding have also deteriorated away. When on a Survivor he claws at them with his free hand while leaning in a direction to cause the Survivors to stumble. The player can try to steer in a better direction, but can't outright resist where the Jockey drives them. When a player is incapacitated, the Jockey may leap off of the survivor.

He has large claws, used to help control the Survivors, and wears a loose vest alongside blue boxers. His approach can be denoted by his loud, maniacal cackling. Like the Spitter, it too appears to have pupils. It was first mentioned here.

In this article, Valve said that the Jockey was made to work in tandem with the Spitter by "steering" Survivors into the Spitter's goo. Valve also said that the Jockey could steer Survivors into fire or even Witches.

One could argue that the Jockey is superior to the Hunter in pouncing isolated Survivors as it can also "steer" them away from the rest of the team at nearly the speed of the Survivors.

Trivia

 * The Jockey has some resemblance to Fritz from the 1931 film adaptation of Mary Shelley's novel, "Frankenstein" (contrary to what people suppose, Igor was not in this movie).
 * The Jockey appears to be halfway into the transformation of a Hunter, as he has developed jumping and "pouncing" abilities, and having a boost of strength while not developing muscles. He also possesses enough strength to knock down a Survivor but instead using its weight to control it. Its fingers also seem to be sharpened so that he can claw Survivors, and the loud screech the Hunter has seems to have been replaced with a loud, maniacal laugh. This hints that the new Special Infected, including the Charger and the Spitter, appear to be variants of the old Special Infected.
 * The Jockey could have used the sound from the Screamer. As stated in the developer commentary, the Screamer has a cackling laugh as does The Jockey.