Werewolves

A werewolf is a human who has been infected with lycanthropy and who changes into a dangerous wolf form when the moon is full.

The werewolf is a most unusual creature in that it doesn’t technically exist except for a brief period of time around the full moon. At any other time, a werewolf is a completely normal human. However, the term werewolf is used for both the wolf-like creature and the normal human. Remus Lupin is a werewolf by definition even if he isn’t in the actual form of the wolf.

A werewolf comes into being when a person is bitten by another werewolf. Once this happens, the person must learn to manage the condition. Modern potion-making has come up with a draught called Wolfsbane Potion which controls some of the worst effects of the condition (PA18). Wolfsbane Potion is quite difficult to make, even for fully qualified wizards, and is said to have a rather disgusting taste (PA8). Nothing will completely cure a werewolf, unfortunately.

A werewolf can be distinguished from a true wolf physically by several small distinguishing characteristics, including the pupils of the eyes, snout shape, and tufted tail (PA9, OP28).

A werewolf when transformed is a fearsome beast indeed. All trace of human awareness is gone and the werewolf will attack any witch or wizard, including the werewolf’s best friends. This transformation is triggered when the moon is full, although there is some evidence that a werewolf who is taking a regular regimen of Wolfsbane Potion will not transform until the moonlight actually strikes him. (Lupin, for example, did not transform while he was in the Shrieking Shack, even though the moon was full, until he stepped outside and the full moon emerged from behind a cloud. Lupin had been taking Wolfsbane Potion for months.)

The Ministry regulates werewolves. Already in 1637 there was a Werewolf Code of Conduct (PS16). Dolores Umbridge was instrumental in the passage of restrictive anti-werewolf legislation that Sirius Black said made it almost impossible for Lupin to get a job (OP14). According to Newt Scamander, werewolves have been shunted between the Beast and Being Divisions of the Department for the Regulation and Control of Magical Creatures for years. At one point, the Werewolf Registry and Werewolf Capture Unit were both in the Beast Division, while at the same time the office for Werewolf Support Services was in the Being Division (FB).

Other canon notes

Discussed in Defence Against the Dark Arts class (PS13, CS10), with an essay assigned by Severus Snape when he once substituted for Lupin, although werewolves weren't due to be covered until the last chapter of the third-year DADA textbook (PA9).

Werewolves have been mentioned several times in connection with Harry's Defence Against the Dark Arts teachers. Professor Quirrell had encountered them in Black Forest (PS5), and at one point discussed in class how to treat werewolf bites (PS). Gilderoy Lockhart, supposedly, once defeated the Wagga Wagga Werewolf (CS10), something that may be discussed in his book Wandering with Werewolves (CS4). Lockhart eventually confessed to Ron Weasley and Harry Potter that an "ugly old Armenian warlock" had actually performed the rescue of a village from werewolves that he himself had taken credit for (CS16). Remus Lupin, of course, is a werewolf (PA18, OP5, OP28).

Draco Malfoy as a first-year had heard that werewolves lived in the Forbidden Forest (PS15), and a year later Ron referred to the same rumour (CS15). The fascinating origin of this rumour can be found in the "werewolves" essay on WizardingWorld (Pottermore) (CMB).

Tom Riddle alleged that Rubeus Hagrid raised "werewolf cubs" under his bed as a youngster (CS17). (This, according to JKR, was a slanderous lie from Tom Riddle, since werewolves don't have "cubs".)

Bill Weasley was bitten and slashed by a werewolf, Fenrir Greyback, in the Battle of the Tower. His face was severely damaged with magical wounds which might not heal properly. Because the werewolf was not transformed into a wolf, he will likely not become a werewolf himself.

While in St Mungo's Hospital, Arthur Weasley mentioned a patient in his ward who had been brought in after being bitten by a werewolf:

"But that fellow over there," he said, dropping his voice and nodding towards the bed opposite in which a man lay looking green and sickly and staring at the ceiling. "Bitten by a werewolf, poor chap. No cure at all."
"A werewolf?" whispered Mrs. Weasley, looking alarmed. "Is he safe in a public ward? Shouldn't he be in a private room?"
"It's two weeks till full moon," Mr. Weasley reminded her quietly. "They've been talking to him this morning, the Healers, you know, trying to persuade him he'll be able to lead an almost normal life. I said to him - didn't mention names, of course - but I said I knew a werewolf personally, very nice man, who finds the condition quite easy to manage."
"What did he say?" asked George.
"Said he'd give me another bite if I didn't shut up," said Mr. Weasley sadly (OP22).