Shagnasty

"Shagnasty" is the nickname that Harry Dresden gives to the Naagloshii, a skinwalker. It is first seen in Turn Coat.

Description
Shagnasty is an evil skinwalker who likes hurting people. It's a capable sorcerer as well as using its own natural abilities. The more afraid you are, the more powerful they get. It has a horrific psychic stench  and seen with a wizard's Sight, it can be debilitating.

It can eat magic, then adds its victim’s power to its own. A wizard would give him a lot of power and that power draws them, especially a wounded wizard. The longer he’s away from home tribal lands, more power it costs him. Only a true shaman of the blood could drive a Naagloshii away by performing an enemy ghost Way.

It's often veiled and invisible. Harry got a brief glimpse when its veil hit Harry's shield: It was immensely tall, lean, shaggy, vaguely humanoid thing with matted yellow hair, overlong limbs and long, almost delicate claws. Its long arms hung bellow its knees. Golden eyes.

It may assume also the forms of: python-shaped blur, badger or wolverine;  giant spider, one of Lara's security guard, a great bear-like something  Gorilla-shape, something feline under a veil;  Fight with Listens-to-wind: shadowy serpents (dozens), wolf-like shape, simian (ape-like) creature, ferret, alligator, falcon with fur turfts, four-legged bat-winged-thing with eyes and mouths everywhere—defeated, it ran off.

Shagnasty is a real Naagloshii. Murphy notes that he isn't exactly an errand boy.

Turn Coat
In Turn Coat, When Harry Dresden was driving home, his senses told him he was being followed. He opened his Sight and saw the skinwalker. Seeing the pure mass evil perched on the roof of a building with his Sight nearly disabled him. He would See it repeatedly with every thought of it. He left his car and walked to Billy's where he used a quiet room to desensitize himself from it. It plagued his thoughts for the remainder of the novel.

While at Billy's, the skinwalker killed Kirby and badly wounded Andi, two members of the Alphas who were guarding Harry while he was incapacitated. When Billy and Georgia in wolf-form double-teamed him, he turned into a hawk and flew away.

Donald Morgan thought the Naagloshii must have followed him when he took the train from Tuscon. A wounded Wizard is easy prey and he'd gain much power by consuming him. Morgan tells Harry of a previous encounter with a skinwalker in which Morgan led it onto a nuclear test range and stepped into the Nevernever, leaving the skinwalker to be destroyed by a nuclear weapon.

While Harry and Anastasia Luccio were talking to Lara Raith at the Raith estate, the Naagloshii attacked. It charges into Lara's office, Luccio zaps it. It hits Luccio and wounds Justine in the belly, exits through the window. The Naagloshii reaches through the floor and pulls Harry down several stories, then chokes him for a while. Lara, and her two sisters Elisa and Natalia Raith engage him. He ends up killing and wounding many. The Skinwalker says he has Harry's brother, Thomas Raith, and demands that Harry trade Morgan for him.

Murphy questions why Harry got stuck with two tails and suggests that Shagnasty—as Harry now calls him—crashed into the Raith estate as a distraction to allow Madeline Raith to make her getaway. Shagnasty was the perps ace in the hole until Morgan escaped and he played his hole card feeling pressured.

Shagnasty tortured Harry's brother, Thomas Raith. He brings Thomas to Demonreach where Harry squares off with him. In Harry's final punch he uses a cord of force mixed with soulfire that nearly strangles him, but Shagnasty gets free by drawing a circle. Before he's kills Harry he gloated, then slashed Harry's face, giving Harry a mean new scar. He taunts Harry about Thomas, hunger-mad being in the tent with Molly. Toot-Toot attacks the Naagloshii with a utility knife, scores twice and runs, the creature shoots a force at Toot knocking into some thorny bushes, where he lay still.

In the end of Turn Coat, it was eventually driven off by Injun Joe, who himself morphed into various animals perfectly suited to defeating whichever animal the skinwalker was imitating. The Naagloshii's current whereabouts are unknown.