Complete Monster/Teen Wolf

Teen Wolf has given us more than a few of these:


 * Gerard Argent is the cold-blooded head of the Argent family and de facto leader of the werewolf hunters. Prior to the series, he ordered his daughter Kate to seduce a young Derek Hale, before killing his family. In his first appearance, Gerard murders an innocent werewolf before declaring all werewolves in Beacon Hills will be indiscriminately killed, even forcing his son Chris to kill . Gerard tries to manipulate his granddaughter Alison into becoming a ruthless hunter herself. Killing the master of a dangerous beast to take control of the monster, Gerard uses it to hold an unwitting crowd hostage. Despite claiming to be avenging his daughter Kate, Gerard reveals his true plan was to get bitten by a werewolf, to cure his cancer and take his powers and declares he would kill his own son for the sake of his survival. When his plans backfires, Gerard simply orders his monster to kill everyone. In the past, when a werewolf tried to negotiate peace between the wolves and the hunters, Gerard murdered both the werewolf pack and his own men, framing the innocent werewolf for the massacre. In season 6, Gerard takes advantage of the spreading fear in Beacon Hills to amass an army of hunters to simply wipe out not just the supernaturals in Beacon Hills, but the whole world as well. In the finale, Gerard rounds up Scott's loved ones with the intent to kill them, and having used Kate, now a werejaguar, coldly attempts to kill her for being "one of them."
 * The Nogitsune possessing Stiles, the Big Bad of Season 3B, mentally and emotionally torments him, forces him to torture his friends, and go on constant murder sprees. When originally summoned decades ago, it was summoned by Kira's mother at an internment camp to avenge her American lover, but instead massacres the internment camp, prisoners and guards alike, before finally being subdued. In the present, he sets up multiple traps, including sabotaging an electric cable killing several people and rendering Isaac Lahey comatose, has an arrow shot into Coach Finstock's stomach and lets a bomb go off in the Sheriff's station, all while pretending to be Stiles in front of Scott. Stiles's best friend, before twisting a sword in Scott's gut. When he's separated from Stiles, he takes control of the Oni soon after killing . He leads the Oni on a massacre at the local hospital and police station. In the final showdown, he plans on having Scott kill Stiles, citing it as the only way he can be defeated, because it sees it all as a game.
 * Theo Raeken, the Big Bad of Season 5, is a childhood friend of Scott and Stiles, seemingly looking to join Scott's pack, Theo is in fact working with the Dread Doctors in their Chimera experiments; having a hand in the abduction, and torturous experimentation of teenagers, and the following deaths. Disturbed since youth, Theo calmly had his sister freeze to death in a river just so the Dread Doctors would transplant her heart into him. After straining Scott and Stiles's friendship (killing two chimeras in the process), Theo uses Hayden's life to try to get Liam to kill Scott for his True Alpha powers and kill Liam himself in turn for them. When that fails, Theo abducts Lydia then Mind Rapes her out of her mind in order for him to resurrect dead Chimeras as his Pack, leaving her catatonic. With the Beast of Gévaudan on the loose, Theo seeks to kill it so he can take its power. To that end, he also Chronic Backstabbing Disordersells Malia out to the Desert Wolf. When he learns how to take power from others, he does just that to two members of his pack, killing them. When all his plans fail, Theo makes a final attempt to kill everyone just out of spite.
 * The Beast of Gévaudan, also from Season 5, is know as history's most famous and vicious werewolf. In the 1700s, he was credited with killing 112 people, but the Doctors claimed it was closer to 500, including children. His identity was discovered by Marie Jeanne Valet to be her own brother, Sebastian Valet. When he confessed, he was proud of his crimes, and wanted to be remembered for the horrors he caused. When his sister opposed him, he was willing to kill her with no hesitation or remorse. In the present day, the Beast is resurrected by the Dread Doctors, but doesn't remember his former identity. As he tries to piece his memory together, he goes on a new killing spree. Once he's resurrected via his vessel Mason Hewitt, he is content with erasing him and kills the Dread Doctors, prepared to pick up where he left off.
 * Garrett Douglas, from season 6A is the assumed identity of Der Soldat, a Alpha löwensmensch werewolf who held the rank of Hauptmann as part of Ahnenerbe. In World War II, he used the Nazi party as a means to his own ends and he learned of the Ghost Riders, which he sought to use to create an army under his control. At one point he kills another officer who mocked his plans and forced one of his men to open the rift into the Ghost Riders limbo like world, burning said soldier to death and resulting Wild Hunt decimating his men. After being kept in suspended animation by the Dread Doctors, he broke free following their deaths, assuming the identity of a harmless high school teacher, while secretly killing people and eating the pineal gland of their brains. When the Ghost Riders arrive in Beacon Hills, he seemingly helps Scott's pack capture one of the riders, before killing said rider and devouring its pineal gland and taking its powers. With his newfound powers, Douglas begins reaping souls, and seeks to brainwash Parish to use his Hell Hound powers to enter the riders world and amass an army from the souls the Wild Hunt claimed. In the finale, he keeps Corey locked up with wires and electronics hooked into his body, using his chimera powers to merge the Wild Hunt's world with the mundane world to bring about his supernatural army. All the while mocking Scott and his friends for standing against him, intending to place them in his army as well.
 * The Anuk-Ite is a malevolent entity that delights in using fear to torment it's victims. The Anuk-Ite spreads fear through supernatural spider infestations, turning fear into murderous desire as communities tear themselves apart, all the while sowing in the chaos and death it causes. Imprisoned in the Wild Hunt, when it escapes one of the first things it does is the head doctor of Eichen House to kill his patients in a panic. The Anuk-Ite plays up the growing fear among Beacon Hills human population, making the genocidal war Gerard is starting even worse to draw power from it and directly influencing most of his soldiers. Taking advantage of the paranoia in the school, it uses Miss Monroe's cruel tests of flushing out supernaturals, to find the one who hosts it's other half, killing those it can't use; it's other half is in a young werewolf named Quinn whose pack it kills, before having Aaron kill Quinn and becoming full once again. Now free, it displays its power by turning everyone who cross its path into statues, before making a deal with Gerard to battle Scott. In the finale, the Anuk-Ite picks off Scott's allies, using hallucinations and their emotions (particularly loved ones) against them, showing a clear understanding of human thought process, and it's revealed the people turned to statues are still alive within them. During its final confrontation with Scott, it mocks Scott with Alison's death, revealing its desire to never be trapped again using Scott's fear and taking his powers.