The Jockey

The Jockey is a new form of Special Infected that will appear in Left 4 Dead 2. It will jump onto the Survivors and cling to their head and upper back. From there, they can steer the Survivor while clawing them. The Infection has caused the Jockey to mutate a large amount of muscle upon his upper back and neck, causing him to hop around in a spider monkey motion. His fingers and toes have also increased in length, adding to his baboon-like profile. His lips and the skin surrounding have also deteriorated away, giving him a more skeletal look at the mouth area. He wears a tattered white t-shirt and blue boxers. He has a gibbering, apelike vocalization, and cackles hysterically whilst attached to a Survivor

The Jockey can use his ropey frame to jump an inhuman distance, however not as far as the Hunter can. When on a Survivor, he claws at them with his free hand while leaning in a direction to cause the Survivors to stumble. Again, the clawing does not do as much damage as the Hunter does. 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.

Like the Spitter, it too appears to have pupils. It was first mentioned here.

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.

Gameplay
When playing as the Jockey, the hands are like the Hunter's, except they are curved downward, and the arms are sticking out straighter. The hands are always twitching and shaking, and he is almost always laughing. While the there is a Jockey on a survivors back the infected can move the control stick to resist the Jockey's influence over it, this only has a slight resistance and will slow the survivor to a walk.

Trivia

 * This is the first Infected which can leap off the Survivors, not having to kill them. He can also hop off after incapacitating a Survivor.
 * As seen in Gameplay Videos, the player can resist the Jockey by walking in the opposite direction. When resisted, the Jockey becomes slower for a few seconds, allowing fellow teammates to reach the player.
 * The Jockey was the first Special Infected to be featured in a video with a Special Infected gameplay.
 * Gameplay footage of the Jockey shows that his "pounces" for his primary attack are much shorter and quicker than that of the Hunters', supporting the theory that all the new Special Infected have less advanced cases of infection. This also means players will have to have good accuracy when attacking, or focus on Survivors distracted by other Infected.
 * In a first person perspective, the Jockey seems to tilt his hands downward like a mantis. His arms also seem to twitch constantly, most likely due to his hyperactive nature.
 * The Jockey is currently the Special Infected with the smallest profile.
 * The Jockey and the Tank are the only two playable Special Infected without the suffix "-er" in their name.
 * The Jockey seems to be a reference to one of the "Mutants" in the Movie "Wrong Turn," as one of them is constantly cackling and also has a hunchback.
 * The Jockey's cries during the period that it performs its special attack sounds similar to that of a ravenous chimpanzee or baboon.
 * When being ridden by a Jockey, the survivor is surrounded by a green light, unlike the red light when being pounced by a Hunter, choked by a Smoker or pummeled by a Charger. That's may be by the fact he's the only Special Infected that you can do something while being attacked (resist).
 * The Jockey bares a slight resemblance to the Clown UCI, only without the make-up.(though his smile is now permanent.)
 * Apparently, to be ridden on by a Jockey is considered humiliating and/or embarrassing to the survivors.