Rugaru

"Rugaru. They're mean, nasty little suckers -- rotted teeth, wormy skin, the works. They start out human, for all intents and purpose. About age 30, they start changing -- bones shift around, animal instincts kick in. But most of all, they're hungry. At first, for everything, but then for long pig. Hunger grows in till they can't fight it, till they got to take themselves a big, juicy chomp and then it happens. They transform completely and fast. One bite's all it takes -- eyes, teeth, skin all turns. No going back, either. They feed once, they're a monster forever."

- Travis on Rugarus to Sam and Dean

Rugarus were carnivorous humanoids which fed on 'long pig' (human flesh), similar to Wendigo.

Characteristics
Rugarus are born and live as human for all intents and purposes, typically being unaware of their condition, but at a certain age they will suddenly begin craving large quantities of food. This condition is not a virus; rather, it is passed down genetically. A person who possesses this gene will eventually give in to the urge to feed and will devour a human. The change starts out with an increased hunger for all food, but quickly transforms into a powerful craving for flesh, specifically human. The change itself is very fast, occurring within minutes of the rugaru's first bite of human flesh. Once the rugaru gives in to its desire to feed on a human, there is no going back; the rugaru will lose its human appearance and change into a monster.

Although it is believed by some that a person with this gene can overcome their curse, no rugarus are known to have been successful in this. Apparently, the final change can be avoided as long as the rugaru resists eating human flesh. Unfortunately, the urge is extremely strong and is practically impossible to ignore.

Behavior
After consuming human flesh, the rugaru will act as a predator, often eating violently and moving on all fours. Their human, emotional side becomes less active, yet they retain all physical abilities. They do retain full memory of their human life and may display some capacity for emotion and empathy, but this is almost always eclipsed by their overpowering need to feed.

Physical Appearance
Rugarus look nearly completely human, aside from several exceptions: Their skin is "wormy" (as described by Travis); it is considerably paler than human skin and has prominent veins and skin folds. Full grown rugarus obtain demonic, pitch black eyes, while unchanged rugarus have red, bloody scleras.

Season 4
Sam and Dean learn of the existence of rugarus when a fellow hunter named Travis asks for their help. He sends them to watch Jack Montgomery, a man who is about to become a rugaru. Jack is showing signs of his impending change but has managed to resist it so far. When Sam and Dean later meet Jack and warn him about what's happening, Jack kicks them off of his property rather than heeding their words.

Later on, after an altercation with his wife Michelle, in which he nearly bites her, he climbs to the window of an undressing woman and shows clear signs of his desire to break in and devour her. Sam and Dean, who have been following Jack, charge into the woman's apartment with blowtorches, prepared to rescue her, but find that Jack has regained control of himself and retreated. Jack returns to his house with the apparent intention of reconciling with his wife and working to resist his transformation, but finds his wife bound by the hunter Travis. Travis quickly incapacitates Jack and binds him to a column.

At first, Jack begs Travis to spare his wife, but after learning that Travis intends to burn both of them alive (owing to the fact that Jack's wife is pregnant, and her baby will inevitably carry on the rugaru gene), his hunger and desire to protect his wife leads him to break free of his bonds. Unable to resist, he sinks his teeth into Travis's neck and swallows his first bite of human flesh, transforming him into a full-blown rugaru. When Sam and Dean try to stop him, he overpowers them, locks Sam in a closet and prepares to eat Dean. However, Sam manages to escape and reluctantly kills Jack with a flamethrower.

Season 5
Chuck Shurley mentions in the afterword of his last Supernatural book's manuscript that Bobby will be hunting a rugaru for a week after Sam had jumped into Lucifer's Cage and stopped the Apocalypse.

Season 6
Sam and Dean capture a rugaru and deliver it to Crowley's demons for interrogation on Purgatory's location.

Season 8
While in Purgatory, Benny Lafitte and Dean captured and interrogated a rugaru who knew where Castiel was. After much threatening, Dean managed to get him to spill the location; and afterwards, Dean stabbed him through the neck with Ruby's knife.

Season 12
While working with the British Men of Letters, Mary Winchester hunts down and kills three rugarus with a special gun made by the British Men of Letters. She was accompanied by Arthur Ketch on the hunt.

Powers and Abilities

 * Super strength - A rugaru is much stronger than a human and indeed most other monsters. They are able to snap bones and break steel chains with ease.
 * Super speed - Speed is important to a rugaru. Being fast allows them to stalk and catch their prey.
 * Super stamina - A rugaru will never tire nor will it give up the hunt once it has begun stalking its prey. However, they do need to eat.
 * Invulnerability - They cannot be harmed by ordinary weapons. Fire is the only thing that can kill a rugaru.
 * Super senses - As a species rugaru have an incredibly strong sense of smell and sight. One was able to catch the pleasant scent of a young woman undressing from a great distance.

Weakness

 * Fire - Fire is the one of the few things that can kill a rugaru. To kill them, hunters need to burn them alive.
 * Demon-killing knife - While trapped in Purgatory, Dean was able to kill a rugaru with the 'demon-killing knife' by stabbing it through the chin which impaled the creature's brain. The use of a knife to kill the rugaru contradicts the fact that fire is the only thing that can kill a rugaru - but it was hardly an ordinary knife.
 * The Colt - Seeing as this gun can kill nearly anything, man or monster, it is probable that it can also kill a rugaru.
 * Death's Scythe - Since it's said it can kill anything.
 * The First Blade - Being it's said that it can kill anything, as long as it is accompanied by the Mark of Cain.
 * Rugaru Killing Gun - The British Men of Letters at some point invented this weapon through a combination of technology and magic that melts the brains of a rugaru and kills it.

Known Rugarus

 * Jack Montgomery's father (deceased)
 * Jack Montgomery (deceased)
 * Montgomery Child
 * Unnamed Captured Rugaru (presumed deceased)
 * Purgatory Rugaru (deceased)
 * Unnamed Rugaru (deceased)

Appearances

 * Season 4
 * Metamorphosis
 * Season 5
 * Swan Song (mentioned only)
 * Season 6
 * Caged Heat (The monster Sam and Dean hand to Crowley's minions)
 * Season 8
 * What's Up, Tiger Mommy? (flashback only)
 * Southern Comfort  (mentioned only)
 * Freaks and Geeks (mentioned only)
 * Season 9
 * Bad Boys  (mentioned only)
 * Season 10
 * The Things They Carried (mentioned only)
 * Season 11
 * The Chitters (mentioned only)
 * Season 12
 * Family Feud

Lore
Rugarus in traditional folklore are completely different from the ones featured on Supernatural. In traditional Cajun / Louisiana and Canadian folklore, a rugaru is a monster with the head of a wolf and the body of a man that prowls the swamps to feed on the blood of humans.

Becoming a "rugarou," as it's spelled by some (itself a variation of the original french loup-garou), is dependent on which lore is referenced. Some become one by being bitten by another rugaru; others change after being cursed by a witch. French lore says that people become a rugaru if they break Lent seven years in a row. Once transformed into a rugaru, they are cursed to be a blood-sucking monster for a hundred and one days and must feed on human blood (not flesh) daily. A rugaru's hunger for human blood is said to be insatiable and he possesses superhuman strength and speed.

Some lore claims that a rugaru can be cured of their condition if someone draws its blood on the end of the hundred and one days. After that, they can't speak about it to anyone until the hundred and one days are over. If so, the victim will return as a rugaru along with the person that cured them. Rugarus in traditional lore are said to fear frogs, are repelled by the sign of a Hexagon and salt, which will burn its skin if it comes into physical contact with it. Just like in Supernatural, rugarus can be killed by fire or by decapitation; but on a full moon, the only true way to dispose of the beast is stabbing it deep in the chest with a wooden stake curved from cypress and dipped in dead man's blood.

Trivia
Rugaru Rugaru
 * Rugarus, along with shapeshifters, are one of the few monsters shown so far that reproduces almost solely by sexual contact with a human. The offspring that results is presumed to always be a dormant rugaru.
 * Although it was never stated in any of the episodes that the rugarus appeared in, it seems that all rugaru are male, and that they only turn after they have impregnated a woman. In the 1st episode that they appeared in, it was stated that the rugaru Jack's father had turned into a rugaru when his wife (Jack's mother) was pregnant with Jack; when Jack himself turns, it is after he and his wife have conceived. Biologically, this makes sense, as it allows at least one rugaru to be present on Earth.
 * Rugarus share certain similarities with Wendigo. Both are defined by an insatiable hunger for human flesh, can only be killed by fire, and were changed from a human state into monsters by cannibalism. In the real world, both legends originate in North America.