The Charger

The Charger is a charging Special Infected that knocks everything in its way. It is a new Special Infected in Left 4 Dead 2, and will be playable in Versus mode. It makes a donkey-like groaning sounds as seen in the first teaser while running toward Coach. It is described as serving to split up Survivors who are grouped up in a small place. While it matches the Survivors' speed, it can charge toward the Survivors, making it faster. However, this will lead to a slow stop and difficulty at turning. It's not as powerful as the Tank, but can still take a beating and do half of its damage by charging. It is also less maneuverable, making it easier to dodge as it charges around. The Charger knocks the Survivors to their feet if they are too close when he charges past them making it harder for the Survivors to free the charged victim

It does damage per hit when its picking and grabbing Survivors up in a "pinned-style" much like the Hunter and Smoker after reaching a wall or other viable hard surface (since the update), and then smashing them repeatedly into the ground, but it does much more damage than the other Special Infected such as the Hunter and Smoker, possibly doing around 10-15 damage per hit on easy mode. This delivers 10 damage per hit on normal difficulty. When pinned by the Charger, Common Infected can still do damage to the Survivor like the Smoker.

In terms of appearance, the Charger is a large Infected, roughly the height of the Tank, and about half as wide. Unlike the Tank however, only one of its arms have been mutated while the other is small and presumably useless. The Charger does not use its arms for movement, as its legs are still capable of supporting it. The Charger seems to prefer using the arm as a shield. Like the Tank, though, it charges through crowds of Common Infected to get to the Survivors. It also sports blue-overalls, meaning one could assume the Charger was once a farmer, especially considering the Southern setting. This could also explain why only one of his arms is so bulky; A farmer working in the fields would use only one arm while cutting crops with a sickle.

Like most other Special Infected, the Charger has a distinctive sound. It does not shake the ground as the Tank does, but itself does seem to be loud, growling and moaning at enemies distinctively.

Note: The following damage is calculated on normal difficulty.

While charging, the Charger will run like a Tank, using its mutated arm to walk in a "monkey" style. It will run faster than a Survivor with full health, and will grab the first Survivor it charges into, holding the survivor out in front of itself while continuing to run through anything (including other Survivors, doing about 8 damage.) until he slams the Survivor into a wall, doing about 10 damage upon impact. The charger itself also appears to receive damage when it hits a solid object, although it will receive less if it has a survivor, and more if it misses and hits a wall. After that, it slams the Survivor in a pinned like style until the Survivor is dead or the Charger is killed. This attack does 15 damage per slam. In a recent gameplay video of the new Scavenge mode, it only takes the Charger 9 seconds to incapacitate Coach, who at the time was in green health. The video can be found here.

Extent of Mutations
The Charger has been heavily mutated by a strain of the Infection. It appears to be halfway into the transformation to a Tank. While not quite as hulking as a Tank, it has an enlarged torso and right arm and an abnormally shaped head. Its left arm is shriveled and hangs oddly off the side of its hunched back. It wears a pair of blue overalls that have slightly burst due to its malformations. While pounding a Survivor, the Charger's useless arm flops around comically. Possibly due to its choice of attack, the Charger appears severely damaged; the creature's scalp and face appear smashed or caved in. Along with this, half of its face is missing, revealing the skull, and the nose has been torn off clean.

Even though it seems like the Charger is half way into a Tank, at the same time it also seems to be turning into an Infected all of its own. The intro scene of Left 4 Dead 2 shows that its left leg is much larger in size than the other, meaning its legs are starting to lose support on it, similar to the Tank, and that the right leg is under much more weight because of its large, mutated arm. This occasion is closely related to how the Jockey resembles the Hunter, except that the Jockey is half-way through transformation and isn't fully crouching. The arm also appears to be formed of tumours and scabs, as opposed to pure muscle that the Tank has, forming a protective layer around much of the Charger's arm and back. Because of this, the Charger is probably undergoing an entirely different mutation.

The Charger could be a Tank that underwent difficulties during its development. In addition, the Tank's lower jaw is consumed by its upper muscle. However, in terms of the Charger, the jaw appears to be largely unharmed and fully functional in comparison to the Tank's. In addition, the Tank possesses two muscular and bulky arms unlike the Charger which sports only one overdeveloped arm accompanied by a undeveloped/limp second arm which could have been drained of most of its nutrients and transferred them to its opposite arm due to unknown development difficulties. When it charges, it uses its right arm to run like a Tank, furthering the similarities between the two.

Behind the Scenes
Before the Charger's design was finalized, it used a reskinned Hunter model so that the team could test out how the Charger would play while the art team was designing it.

Trivia

 * Concept art shows that the developers had planned for the Charger to be either a large zombie with an armored upper skull yet tiny 'T-rex arms' or a 'tripod' dog-like animal.
 * Originally, the Charger would stop immediately when it had grabbed the closest Survivor; now, it continues charging until it hits a wall, then starts slamming. However, this is most likely the developers testing different ways the Charger can attack, as this would unbalance gameplay and render the Smoker and the Jockey useless. If given this ability, he would be able to pull a Survivor extremely far from its team or even kill them by dropping them off a cliff.
 * The Charger bears a striking resemblance to Bandersnatch from the Resident Evil franchise. It also appears similar to the build of the Flood Tank Form from the Halo series.
 * Sounds used by the Charger at Comic Con are re-used ambient sounds from various sections of Ravenholm in Half-Life 2. In addition, the Charger's roars and grunts seem to be remixed or altered versions of Half-Life 2's Poison Headcrab Zombie.
 * The Charger's model appears to have been changed again, now more closely resembling its skeletal-faced concept art. Its ramming arm now seems to have become not just bigger, but rock-hard as well, however, this could just be one of the few skins the Charger will use. It can be seen here.
 * Charger seems to have the most distinctive sounds in game, some of them have been mistaken as words, such as "Don't shoot me!"
 * The Charger shares extreme similarities to the Big Daddy Bouncer from the Bioshock franchise both aesthetically as well as in terms of ability. The Charger and Bouncer both share the ability to inflict large amounts of damage by tackling their enemies and lashing them with their bulky right arm. In addition, the Bouncer and Charger both feature the unique ability to shake a player(s) camera upon being tackled by them.
 * The Charger also has a music cue when getting pummeled, the music can be found at the end of this video http://www.youtube.com/watch?v=2FyNc57Fsks