Beast

Jackie sat up in bed, as if shocked by cattle prod. A shape slid past his window. A scratch came at the door to her one room cabin. She cautiously opened the door. A tiny wolf pup whimpered at her from the snow. 
"Awww. Come here. You look cold."
She picked up the pup and dried her off. She carried her to his bed and put her down on the pillow next to hers. Without thinking anything else about it, she went back to sleep. 

When Jackie woke up the next morning, the pup was gone. In it's place was a full-grown she-wolf. Jackie started to panic, but the wolf only licked her hand and whined. She got up and brought the wolf a piece of steak. It gobbled the food up. 
Jackie went about her daily chores. Everywhere she went, the she-wolf followed her. This went on for several weeks. One night, the she-wolf changed into a human with wolf ears, fangs, and tail. 
"You have been kind to me. Therefore, I will give you a gift. I will move my lair here, and you will be my second in command. I wish I could keep you as you are, but that is not possible."
She softly bit her hand. "This is the main part of my gift."
Jackie's whole body burned as it changed. Dark brown fur started growing in between her legs. It crept down her legs and covered her feet. Her feet cracked as they shifted into claws. Her ankles moved up and she grew several inches. A tail ripped through her jeans as her feet grew and tore apart her hiking boots. The fur crept it's way up her torso. It covered her boobs and belly in snow white fur. It covered her arms. Her nails became razors and her hands grew. Her eyes changed and became orange. Her ears became pointy and moved to the top of her head. White fur covered her face. Her teeth became jagged points. Her face elongated and her hair became a wild mane. She grew several feet. Muscles bulged to the point of insanity.  Her clothes shredded into loose strips that hung off her body. But with the change came extreme, cruel intelligence. Her mistress bit her one last time. 
"Here are your powers, my beauty."
Jackie growled her thanks. Her claws and fangs glowed blue as the magic rushed through them. Her boobs and butt grew several sizes larger to compensate for her growth. Her muscles bulged one last time. She howled. A beast had been unleashed in the forest.