Werewolf

"What about a human by day, a freak animal killing machine by moonlight don't you understand? I mean, werewolves are badass. We haven't seen one since we were kids. You know what the best part about it is? We already know how to bring these suckers down. One of these bad boys right to the heart."

- Dean, about Werewolves

Werewolves, also known as Lycanthropes, are a race of partially-human supernatural creatures, able to morph into a combination of human and wolf, that have been encountered by hunters such as Sam and Dean Winchester. It has been stated that werewolves and skinwalkers are cousins. There is no known cure for lycanthropy.

Characteristics
Werewolves are humanoid carnivores, the ultimate combination of wolves and humans. They feed on prey found nearby, which, due to the increase of urban areas, are mostly people. When they devour the flesh of victim they leave a recognizable trace on the prey: they always eat the heart, although they do sometimes eat more of the body. The transformation only occurs after a werewolf bites a victim, the way werewolves continue the spread of lycanthropy. However, they are also capable of having children who are born werewolves.

In their werewolf form, their bodies change rapidly, changing into a fitting definition of predators. Their nails increase to sharp, long claws, fangs increase in size and eyes resemble those of a wolf rather than of a human.

Types of Werewolves
There are two types of werewolves shown in the series: regular werewolves, and pureblood werewolves.

Regular Werewolves
Regular werewolves can only turn into a werewolf during nights coinciding with the peak of the lunar cycle during their sleep - as Sam and Dean waited for Madison from turning into a werewolf again by remaining in her apartment during a night under a full moon, and no werewolf transformation was seen while she was awake. However, when the next night arrived, Madison fell sleep and went on another feral rampage.

In the series, the people affected by the disease have a short-term memory loss, often leaving them in confusion of wounds which might have been caused during the night. In the series, Dean manages to cut Madison with a silver knife, making her wonder how the wound was created.

Despite having no control over and during their transformation, certain fragments of their personality and fears can be imbibed by the werewolf's mind, especially strong ones. Aside from hunger, suppressed fears or aggression can drive them to kill in their beast form. As such, a werewolf's attack may be controlled by these emotions, depending on which one is dominant. When Madison was attacked, she was turned into a werewolf when her attacker felt an intense drive to mate. On another hand, Madison defensively killed a stalker when she felt that the man was becoming too invasive with respect to her privacy.

Pure-blood Werewolves
Werewolves that are closer to the Alpha up to the fourth generation are referred to as Pure-bloods. Their human side retains more control over their animal side, and they can remember afterwards what happens during their transformation. They can transform any time they want. Pure-bloods can also access some of their werewolf powers such as enhanced strength and speed while in human form.

Pure bloods can also feed on just animal hearts, but if they get a taste of human hearts, it is harder to control themselves. They will be drawn to eating human hearts, the temptation escalating with each heart eaten. It is still difficult for them to eat animal hearts even if they can resist (one stated he had to force himself to swallow them). It's indicated that pure bloods are both those bitten by a first, second and third generation werewolf and those who are offspring as it has been shown werewolves can have children and one werewolf existed that was fourth generation.

Despite purebloods having control over themselves in their transformed state, being turned into one has been shown to still have a potential detrimental effect on a person's mental state. In the case of Corbin Tilghman, he was transformed from a loving husband into a remorseless killer while Brian Wilcox, already mentally unstable, went completely insane after being transformed and feeling the new power it gave him. The power also changed Tasha for the worst after Kate turned Tasha to save her life.

According to Bess Fitzgerald, its harder for purebloods who are bitten to control their instincts than purebloods who are born as werewolves.

Silver Immune Werewolves
In "Bobby Singer's Guide to Hunting" Bobby mentions a hunter that came across powerful werewolves, that had a more wolf-like appearance and an immunity to silver weapons. The hunter mentioned that they weren't immune to a chainsaw however.

Powers and Abilities

 * Shapeshifting - Werewolves transform into a wolf-like creature only under a full moon. During Season 6, Sam tells Dean that werewolves no longer change under a full moon, but under half moons. The demon Crowley agreed with Sam by saying "werewolves changing under a full moon, so... 09". It is presumably due to the upcoming arrival of Eve. Following her death, werewolf transformation patterns appeared to have shifted back to full moon only, as Sam and Dean were later surprised to be dealing with werewolves outside the full moon. Pureblood werewolves can shift at will, however, they somewhat lose control whenever they feed on human hearts.
 * Infectious bite - While in beast form, a werewolf can turn another person into a werewolf through a bite.
 * Claws and Fangs - Werewolves have powerful claws and are able to cut through solid walls. Their fangs can tear human flesh and can rip out a human heart right out of their chest cavity. Pureblood werewolves are able to transform just their claws without transforming fully.
 * Supernatural Strength - A werewolf's strength is much greater than a human. They can easily overpower humans and can rip flesh and bones with amazing force, their strength is notable even for monsters, as they have been observed as being able to smash stone with their bare hands. Even while in human form, werewolves have incredible strength, as one newly-bitten werewolf saw no challenge in lifting an adult woman high above his head using only one hand, and would accidentally rip a door off its hinges also using virtually no effort. All werewolves seem to have a similar level of strength, however a possible exception to this would be the Alpha Werewolf, as Alphas are naturally stronger than their descendants.
 * Enhanced Agility - Their agility is greater than humans, often allowing them to perform incredible jumps and sprints easily.
 * Super speed - A werewolf can move very fast; they are able to move from place to place in seconds.
 * Super stamina - Werewolves do not tire easily.
 * Super Senses - All werewolves are able to see better than humans in darkness, similar to actual wolves, their sense of smell and hearing is equally enhanced.
 * Invulnerability - Werewolves can't be killed by conventional means and weapons. Silver however, can kill them.
 * Regeneration - Werewolves can regenerate all non-lethal damage, purebloods are even able to completely heal their original bites. Werewolves can even heal damage that would be fatal to humans such as injuries from a car wreck. Even if they are turned into a werewolf after suffering such an injury, they can heal.

Vulnerabilities

 * Silver - A werewolf's only known weakness is their vulnerability to silver, as they can be killed with a silver knife or bullet. Contact with silver will hurt them.
 * The Colt - As the colt has the ability to kill almost anything, it is most likely a weakness to werewolves.
 * Vamptonite - Edgar states the Leviathan additive will kill werewolves as well as vampires.
 * Dismemberment - Kate is able to kill Brian Wilcox by ripping him to shreds. It is highly likely that severe wounds caused by a werewolf, at least its infectious bite, can't be regenerated by other werewolves.
 * Angels - Angels can easily kill any monster including werewolves
 * Leviathans - Leviathans can effortlessly kill any monster
 * Scent Misdirection - Werewolves have been shown to be able to have their scent fooled which allows an enemy to sneak up on them.
 * Decapitation - Mary Winchester killed a werewolf in 1980 by blowing his head off with a shotgun.
 * Wolfsbane

Known Werewolves
There have been twenty one werewolves on the series:
 * Alpha Werewolf (presumed deceased)
 * Madison (deceased)
 * Glen (deceased)
 * Professor Ludensky (deceased)
 * Michael Wheeler (deceased)
 * Brian Wilcox (deceased)
 * Kate
 * Garth Fitzgerald IV
 * Bess Fitzgerald
 * Joy Myers (deceased)
 * Reverend Jim Myers
 * Sheriff Pat (deceased)
 * Russ (deceased)
 * Jobah (deceased)
 * Violet Duval
 * Julian Duval
 * Tasha (deceased)
 * Brandon (deceased)
 * Travis (deceased)
 * Rose (deceased)
 * Corbin Tilghman (deceased)

Appearances

 * Season 1
 * Wendigo (mentioned only)
 * Shadow (mentioned only)
 * Season 2
 * Heart (first official appearance)
 * Hollywood Babylon (mentioned only)


 * Season 4
 * It's a Terrible Life (mentioned only)
 * The Monster at the End of This Book (mentioned only)
 * Sex and Violence (mentioned only)

Madison
 * Season 6
 * All Dogs Go To Heaven (mentioned only)
 * Season 8
 * Bitten
 * Southern Comfort (Mentioned only)
 * Man's Best Friend With Benefits (Mentioned only)
 * Season 9
 * Sharp Teeth
 * Bloodlines
 * Season 10
 * Paper Moon
 * Season 11
 * Red Meat

Trivia

 * Have been mentioned more often than any other monster (13 times), even vampires, but have only appeared 6 times.
 * The brothers have spared 5 monsters over the course of the show, 4 of which were werewolves.
 * Werewolves' depiction in anime is slightly different: Though at first werewolves depicted as human-like creature with claws, teeth, and eyes similar to that of wolves like original TV Series version's, it was an incomplete transformation: The fully transformed werewolf is more like anthropomorphic wolves with brown fur and more wolf-like head, depiction which more faithful with their traditional depiction in myth and popular culture
 * When killing werewolves, the Winchesters tend to kill them with silver knives more often than silver bullets.