Disclaimer: Supernatural and its characters are Kripke's brainchild. I make no claims to the contrary.

Set in the middle of Season 3.

Dean barely needed to aim anymore. Such was reality for a veteran hunter. He had been trained so well and hunted for so long that, more often than not, his handgun found its target without any conscious thought.

Two squeezes of the trigger. Two loud gunshots. Two wounds in the beast's back.

The monster over his little brother stilled immediately, back arching slightly, a high-pitched yelp lancing through the freezing night. Dean's feet were instantly in motion, closing the 25-foot distance as he leveled one more shot—just in case—but it wasn't needed. On the ground, Sam's scrabbling hand reached his own handgun and raised it at the dark figure looming above him. One more shot cracked the night, and the thing—whatever the hell it was—fell backwards to the ground.

"Sammy!" Dean's feet hesitated for the barest of moments, instinct carrying him toward his brother; but years of experience kicked in. He spared his brother a quick glance, but he forced his attention again on the monster crumpled on the ground by Sam's outstretched legs. He needed to make sure the thing was dead first, before it could catch them off-guard. Again.

Aiming his handgun down at the black-skinned, hairless monster, he examined it for signs of life—breathing, or moving, or whimpering, or even the final throes before death—but the creature lay completely still. He huffed out a breath of relief, tinted slightly with apprehension, his breath billowing out around him.

"Well, I guess silver bullets still work on these sons of bitches," he observed out loud, staring at the thing's shiny, black eyes, completely still and lifeless, reflecting the full moon above.

"Yeah," Sam huffed tightly from the ground.

The tone of his brother's voice immediately ripped Dean's attention from the monster. In a single movement, he safetied and holstered his handgun at the small of his back as he spun around toward his brother.

The full moon above provided ample light to see his little brother's face pinched tightly in pain.

"Sammy!" Dean dropped to Sam's side, worry flooding through his chest. "Hey, you okay?"

Sam pulled in a seething breath between his teeth. "What the hell was that thing?"

Dean glanced back over his shoulder at the...thing. It had dry, dark, mottled skin that looked like it might start peeling at any moment, and not a single hair anywhere on its body. Its frame was almost human, but also a little canine at the same time. Now that it wasn't moving, Dean could see that it's face was the slightest bit pointed—snout-like—and it had a mouthful of teeth exactly like a dog's. Looking further down, the thing's arms were mostly human, but also not, and he couldn't quite define why. But it clearly had human hands. Except, of course, for the three-inch, razor-sharp claws.

The thing actually looked like some of the illustrations he'd seen of werewolves in the lore. But he knew what a werewolf looked like in real life, and this wasn't that. He didn't know what this thing was. He shook his head as he stared at it, wide-eyed, trying to swallow the additional worry of not knowing. Finally, he forced himself to answer, "I have no fucking idea."

Sam's answering grunt might have been acknowledgment, but it was laced with a hefty dose of pain, and that's what finally forced Dean's eyes away from the monster and back toward his brother. Sam was straining his head back into the ground, eyes pinched closed, teeth clenched so hard that Dean though he might break them. He leaned forward over his brother again, reaching to grip his arm to draw his attention.

"Sammy? Hey, come on, man, talk to me. Where's the pain?"

"Guh!" Sam jerked on the ground as Dean gave his arm a shake, his other hand shooting across to push Dean's hand away. "Shoulder, shoulder!"

Dean let go of Sam's arm immediately and leaned away as Sam groaned roughly beneath him. He started to apologize, but before he could get the words out, Sam looked up at him with an expression so intense that they died in his throat. Instead, he fixed all his attention on his brother as Sam drew in another breath to say whatever he was about to say.

"My leg," Sam gasped, even as he gripped onto his shoulder—and that simple contradiction had Dean looking down before Sam managed to bite out the rest. "Bastard got my leg. There's—there's something wrong."

Dean started pulling a small flashlight from the pocket in his coat, but by the moonlight, he could already see the gleam of blood on Sam's jeans. As he clicked the light on and aimed the beam, he felt his chest hitch in panic.

"Oh, shit."

Law of Parsimony: the philosophical or scientific principle, according to which an explanation is made with the fewest possible assumptions.

Also known as Occam's Razor or the Precedence of Simplicity: of two competing theories, the simpler explanation is to be preferred.