"But you know how much I like kids, Dumbledore."
-- Fenrir Greyback (HBP27)
Fenrir Greyback was a werewolf, low-level Death Eater, and friend of the Malfoy family. He killed for fun and especially enjoyed infecting children. Greyback was a follower of Voldemort since at least 1970s, though he was in it for the access to victims, not for ideological reasons. It was Greyback who contaminated Remus Lupin as a child (HBP16).
Described as a “big, rangy man with matted gray hair and whiskers.” His black Death Eater’s robes are “uncomfortably tight” and his dirty hands have “long yellowish nails.” His voice is raspy and bark-like. Even when human Greyback has pointed teeth (HBP27). Greyback does not have a Dark Mark on his arm (DH23).
According to Lupin, “Fenrir Greyback is, perhaps, the most savage werewolf alive today. He regards it as his mission in life to bite and to contaminate as many people as possible; he wants to create enough werewolves to overcome the wizards.” Greyback believes that “werewolves deserve blood” and preaches to fellow werewolves that they should revenge themselves upon the wizarding world (HBP16).
Lupin’s father, Lyall, first came across Greyback while working for the Department for the Regulation and Control of Magical Creatures in the 1960s. Two Muggle children had been killed, but Greyback denied killing them, and swore he was merely a tramp and not a wizard. His name was not on the Werewolf Register and he had no wand, but Lyall Lupin thought there were signs in his appearance that he was a werewolf, and wanted to keep him locked up him for 24 hours until the full moon. He was overruled by others in the department who laughed at him, which caused Mr. Lupin to lose his temper and exclaim that all werewolves were “soulless, evil, deserving nothing but death” (Pm). Greyback overheard this, but before a memory charm could be placed on him, he escaped with the help of two other werewolves. He wasted no time telling others in his “pack” that Lyall Lupin had insulted their kind, and in revenge, Greyback attacked five-year-old Remus as he slept during a full moon, turning him incurably into a werewolf (Pm).
Greyback has been a Death Eater since at least 1970s, though he is in it for the access to victims, not for ideological reasons. According to Lupin, “at the full moon, [Greyback] positions himself close to victims, ensuring that he is near enough to strike. He plans it all.” His primary value to Voldemort is as a threat to hold over parents — Voldemort obtains cooperation from parents worried that he will “unleash” Greyback on their children. Indeed, it was Greyback who contaminated Lupin as a child when Lupin’s father offended him (HBP16). Greyback is a lower-level Death Deater — he does not have the Dark Mark on his arm (DH23).
In April 1997, Greyback killed the 5-year-old brother of the Montgomery sisters when he “got carried away.” (HBP22)
The night that Dumbledore was killed, Greyback attacked Harry, but Harry (or someone unseen) petrifies him in time (HBP27). Bill Weasley wasn’t so lucky. Greyback nearly killed him and mauled his face so badly that Madam Pomfrey could not restore it. Werewolf wounds are “cursed wounds” and never heal completely.
Greyback was also one of the snatchers who caught Harry, Ron and Hermione when they were on their Horcrux quest and took them to the Malfoy mansion. Greyback was wounded but not killed during the Battle of Hogwarts by a crystal ball thrown by Sibyll Trelawney. Later, Ron and Neville were able to “bring him down” (DH36).
Leader of the Werewolves, worked as a "Snatcher" for Lord Voldemort.
Fenrir='Fenriswolf,' 'Fenrisulv,' 'Fenrisulf' the gigantic wolf of the God Loki in Scandinavian mythology. He was the brother of Hel (Queen of the Underworld) and when he gaped one jaw touched earth and the other heaven (Brewer's Dict.).
Fenrir is played in the movies by Dave Legeno IMDB
Draco and Fenrir: It was established early in the Harry Potter books that Draco Malfoy is afraid of werewolves. When he and the Gryffindors served detention in the Forbidden Forest, Draco said "The forest? ...We can't go in there at night -- there's all sorts of things in there -- werewolves, I heard" (PS15). Of course, at that time there were no werewolves at Hogwarts - that would happen in third year when Professor Lupin arrived, and Draco said at the beginning of the year rather prophetically, "God, this place is going to the dogs" (PA6). Considering the uproar Lucius Malfoy caused that year when Draco was scratched by a hippogriff (PA6, PA11), it's no wonder that Lupin had to resign when his condition as a werewolf was known among the students (PA22).
While there was no specific canon about why Draco is terrified of werewolves, it's possible his parents had told him or warned him about Fenrir Greyback, since Lucius was a Death Eater and would have seen Fenrir up-close-and-personal more than once. Rather ironically, Draco dropped Fenrir's name as a "family friend" to scare Mr. Borgin in Knockturn Alley (HBP6). Yet when Fenrir emerged on the Tower just before Dumbledore's death, Draco seemed repulsed by him, trying not to look at him, and vowing that he never invited him there (HBP27). It's just possible that Voldemort sent Fenrir purposefully to put pressure on Draco to kill Dumbledore, since the Dark Lord was quite good and spotting the deepest fears of his Death Eaters. Having a werewolf that killed for fun outside of the full moon and had blood dripping from his teeth was a special kind of persuasion indeed. And Draco was not just afraid for his own life, but for his parents as well.