Worgen are half human, half wolf creatures that can be found in various parts of Azeroth. Anyone familiar with the Shadowfang Keep dungeon, or the Worgen in the Woods quest in Darkshire, will know, and probably fear, Worgen. They are a notable mob for skinners, as they can be skinned, and also drop cloth and money.
They are humanoid. Hunters can only tame beasts. Despite their wolf-like appearance, they cannot be tamed. There is one exception. Garwal is a wolf, and therefore a beast. He is tamable by hunters. When his life reaches 50% he turns into a Worgen. He is the only known tamable Worgen.
Taming the Worgen form of Garwal is not as easy as fighting him until his health is below 50%, then taming him. If he is in either wolf or worgen form, when tamed he will revert to the wolf form. When he takes the form, he has a buff that turns him into the worgen. If he is tamed at the exact moment this buff is applied, he becomes "stuck" in the form. He will act like a wolf, and have the wolf icon when stabled. But he will appear to be a worgen.
There are some specific details that have to be met in order to tame him. First of all, he can only be seen while doing the quest Alpha Worg. In order to tame him, this quest must be in the hunter’s quest log, and cannot have previously been completed. This is part of a quest chain. For the Alliance, the quest chain starts with The Cleansing, from quest giver Watcher Moonleaf, at Steel Gate, Howling Fjord (coords 30, 29). For the Hoard, the quest chain starts with Find Sage Mistwalker, from quest giver Greatmother Ankha, at Camp Winterhoof, Howling Fjord (coords 48, 10). Complete the quests in the chain until Alpha Worg, which gives the task to kill Garwal.
Garwal can be found in the north-west part of Howling Fjord (coords 27, 8). He is a big, gray wolf, and he wanders the area. Currently, he can be found easily due to a group of hunters trying to tame him. Hunter etiquette says wait your turn. Players will go in a rotation, each getting a chance to tame him. If taming him does not work out, don't kill him if you can help it, feign death, and let the next person try.
He has about 14,000 health, and turns into a worgen at around 7,000 health. The strategy is to work down his health with a damage over time (DoT), or with the help of another player, while the hunter tames him. Time it so that his health ticks to 7,000 at the exact moment the Tame Beast finishes channeling. They must overlap at the exact second for the worgen buff to stick. If done correctly, he will actually transform into a worgen just after the taming is complete.
Timing varies by the hunter's level and damage output. This will take a number of tries to figure out (except if blind luck kicks in first). The popular strategy for level 80's is to work down his health until about 56% (7,800 health), then use Wyvern Sting. This will put him to sleep for 30 seconds. Watch the time remaining on the sting. When there is about 14 seconds left, start using Tame Beast. When he wakes up, in the last couple seconds of taming, Wyvern Sting has a DoT causing nature damage. If timed correctly, it will give a tick of damage to bring him to the right level when Tame Beast finishes. It may also be possible to use Serpent Sting (if you are not Survival spec and don't have Wyvern Sting).
It has been reported that entering an arena with this pet will reset him to wolf form. If you are lucky (or skilled) enough to tame him in worgen form, do not take him into arenas. So far, the form does not seem to be broken by leveling him, when he dies, stabling him, or loading other zones besides arenas.
This appears to be a glitch in the game. There is speculation and fear that Blizzard will fix this glitch. This might mean that all of the Garwals out there will be reset to wolf form. Or, as a bonus to those hunters who already have him, they may let them keep the form he is in, but not allow other hunters to capture him this way. Or, maybe they won't care, as it is only cosmetic. It is highly recommended you catch him as soon as possible (Tuesdays are usually when they reset servers and clean up bugs), and don't get too attached to him, in case they do reset.