Cynophobia- fear of dogs.
She'd grown up with pets of all kinds, dogs, cats, fish, mice, snakes; but she'd always had a thing when it came to dogs. When they'd bark at her on the street or sit with her on the couch, something in her stomach turned uncomfortably. She'd never even been bitten by a dog, and her dogs at home were loyal companions, but something about the way they looked at her made her squirm. She loved animals, dogs included, as long as they kept a safe distance.
She'd never been able to put a finger on the exact feeling she got around dogs, until the Tobias case. When Reid went around back, deciding to split up, she knew it was a bad idea. Her field training had told her that the number one rule was to never separate; yet she found herself alone. Slowly opening the door to the barn, she felt her heart rate pick up when Reid was no where to be found. "Reid," she whispered into the darkness, half expecting to see Tobias jump out of the darkness. Instead, she was greeted with silence.
Unsure of how to proceed, she cursed Reid's brilliant idea to leave her alone, and she wished more then anything that Morgan and Hotch would somehow appear and have her back. She stood in the middle of the barn, trying to look everywhere at once. That's when she heard it, a quiet growl from the darkness. Spinning around, she found herself face to face with her biggest fear. The dogs seemed rabid, crazy, and she really didn't want to hurt them. They left her no choice though, when they lunged at her. She managed to kill two before the third one took a bite out of her arm, then she finished off the last one. Pushing the dead weight off of her, she stumbled to her feat.
That's when she saw it, the body; or what was left of it at least. Mangled and destroyed, she never stood a chance against the dogs. Unable to look away, she stared at the bloody mess; finally snapping out of it when the barn door burst open again. "FBI, don't move!" She screamed, pointing her gun blindly at the figures coming in.
She recognized the voices telling her to calm down, but she couldn't bring herself to believe that they were really here. She was finally not alone, and she had never been more happy to see Morgan and Prentiss. Prentiss escorted her carefully out to the ambulance where her are was bandaged. Everyone's focus wasn't on her though, because Reid was no where to be found. She needed to put a lid on her fear for now and find her best friend, because he needed her right now.
She found the bathroom and splashed some water on her face, hoping it would help her control herself. That was the first time she hallucinated a dog behind her, and she had nearly shot Prentiss because of it. She knew Prentiss knew then, and yet she remained the ever loyal friend, "JJ, relax." Logically, she knew that dogs weren't following her, but she just couldn't come to believe that they weren't out to get her.
It had been years since that day, when she finally identified her biggest fear and was forced to confront it all at once, but she remembered it like it was yesterday. Reid was home safely, and he'd overcome his troubles that had steamed from that day, but she never seemed to be able to. She still saw the dogs behind her when she looked in the mirror, and her dreams were still full of their angry growls. She learned to accept it though, and ever since she met Will, it was much easier to deal with.