Sam smells him during his first heat and the both of them are more than a little surprised. Alphas and omegas usually present with their first rut or heat, but when Dean had passed his twelfth, thirteenth, fourteenth birthdays without presenting as an Alpha, their father had slightly less proudly proclaimed him a Beta. Case closed.
Except that it obviously isn’t because Dean’s standing across from him, smelling like slick and sweat and pheromones, and Sam wants nothing more than to knot him even though Dean’s most definitely still able to kick his ass, dripping slick all over the carpet or not. Sam doesn’t even bother suggesting it. He has some semblance of self-preservation left after all.
So Sam does what Sam does best: research.
Knotting, according to three journal articles and four peer-reviewed research essays, is something that Omegas need during their heats, regardless of whether or not there’s a true Alpha present. It would have been great news in a few years after Sam’s gone through puberty and become true Alpha, but he’s still too small to be of any use now.
He can hear Dean sobbing in the other room, the knotted cock he bought the other day not enough to help Dean now that his heat has reached its peak. Toys really only help for so long, and all of pained whimpers crying for relief of any kind makes Sam’s Alpha howl for action. It’s practically ready to just barge into the damn room and take Dean by force, but there’s more than enough proof that an underdeveloped Alpha won’t be able to do much to alleviate an intense heat. Sam’s big enough to have popped his knot, but a new Omega in heat can’t be sated by a recently-presented Alpha, their knots too small and their come too weak. It can make the cravings worse if the Omega isn’t satiated properly first, and that’s the last thing Sam wants to do.
But with the hoarse crying in the other room making Sam’s inner Alpha rattle in its cage, he’s ready to try almost anything. Teeth on edge, Sam pushes himself through another hour of research before he’s got a plausible solution, a last resort really.
He’s out of his chair in seconds, doesn't care how crazy it sounds.
One look at the phone book, and Sam finds someone who delivers. $140. Cash up front. Pick up and drop off. Clean (with the papers to prove it). Sam's sold.
He doesn't give a shit if Dean agrees. He can't take his brother's sobbing for another night. The loud keening pounds into his head, making him want to claw at his ears until it stops.
What's worse is how hard his cock gets every time Dean whimpers in the bedroom. Just imagining the tear tracks down his brother's tanned cheeks makes his knot swell. He’s had to shower three times already, hard and sweaty with the effort of holding back. It doesn’t matter how many times he comes. With his brother heat so damn close to him, his inner Alpha forces his cock hard, showing off his virility to the fertile Omega in the other room.
Fuck, it's not right. It won't be right until Dean's knotted and sated and the heat is finally fucking over so they can both get back to their normal lives.
And if a traitor part of Sam's mind whispers to him that watching Dean hanging off a thick knot is more than just his duty, he tells it to shut the hell up.
The man arrives in under an hour—an hour of Dean whining and panting and moaning behind a semi-closed door where the smell of fresh slick permeates the air. Sam's rock hard by the time he opens the door, hands over the cash, and is the proud new renter of an Alpha stud.
If he knew more about dogs he’d be able to tell the breed, but all he knows it that the mutt is big enough that he doesn’t need to worry about the knot being too small. Hell, maybe it’ll even be too big.
"Dean?" he calls into the room.
He's answered with a half sobbed, "Sammy?"
Sam opens the door all the way and leads the the pup inside, unhooking the leash to let him roam.
"What-" Dean hiccups. "What are you doing? What’s—?"
"He's here to help, Dean."
"No." Dean shakes his head vigorously, but his hips betray him. Dean sways on the bed, practically writhing as his instincts kick in at the scent of an Alpha.
"Yes, Dean. I can't take it anymore. You need a knot, and I'm not big enough yet."
"No," Dean says adamantly.
" Dean ." The word is sharp, and Dean responds immediately to the tone. His entire body stills, eyes cutting to face his little brother, expression meek and willing.
"Get on your hands and knees. We're doing this."
Fresh tears fall from Dean's eyes, but he doesn't protest. He settles on all fours, legs parted to display his slick wet hole to his brother.
Sammy licks his lips as he rubs a gentle finger against him.
Dean's rim parts easily for Sammy, his heat opening him much better than any other preparation could have. Slick pours out of him with each thrust of tiny fingers inside his hole. The dog whimpers beside him, drawing Sam's attention away from his big brother. Sam pets a hand down the mutt's flank and gives the signal that the owner had quickly shown him.
Two taps to the small of Dean's back and the dog helps himself to the slick dripping down his thighs to the mattress. He starts at the knees and works his way up, tongue flicking rapidly over Dean’s hot skin, trying to slurp up every drop of slick. Dean’s panting, thrusting, fucking the open air while he’s cleaned up. By the time the mutt makes it to Dean’s hole, half-tortured sounds are clawing their way from his throat.
Sam’s eyes are glued to the sight. The dog’s tongue isn’t reserved enough to lick inside of Dean with any amount of real accuracy, but he more than makes up for it with sheer enthusiasm. Sam watches the tongue flick over Dean’s puckered entrance, catching slick just as it escapes, and he has to bite his lip to stop himself from shoving the dog aside and helping himself to a taste.
Sam can’t take it anymore.
With another signal, the mutt hops up and mounts Dean.
Two large paws rest on Dean's sides, holding him in place as the pup ruts against his ass. The thick pink cock unsheathes itself and slides through the slick dripping from Dean's hole.
The man had said it might happen, and Sam licks his lips once more before he reaches out and helps the dog's cock enter Dean's hole. It’s softer than he thought it’d be, but Sam knows that it won’t stay that way for long, that it’ll get bigger and bigger until Dean’s fully knotted.
As soon as Dean's tight heat envelopes the dog's cock, two things happen.
One, the mutt picks up speed, cock slamming in and out of Dean's hole at a rapid pace. If not for Dean's over-slicked ass, Sam’s sure the pup would be destroying his brother.
Two—god, Sam couldn't keep his eyes off it—he knows Dean can feel the stretch as the dogs cock thickens with the friction. Dean’s keens get louder, and his rutting becomes more desperate until he’s forced to drop onto his forearms to keep himself upright.
And the moaning… They aren’t unlike the moans that leaked into the living room for the last few days, but they’re much more than those ones. Sam can see these ones. He can see how Dean's back arches, head thrown back even as he bows lower, submitting to the mutt. His hips press back, trying to take the rapidly-thrusting cock deeper inside of him.
The moans escaping Dean's throat are interspersed with hoarse whimpers and keening that drive the mutt wild.
Sam’s inner Alpha makes itself known, crashing into the well-built cage with a shocking amount of force. Sam sees the dog’s knot swelling, the thick bulb at the base of the cock growing larger and larger as he continued to thrust, and the Alpha forces a growl from his throat at the mutt trying to knot his mate.
His own cock thickens, trying to obtain superiority by competing with the still-growing cock inside of his brother. It doesn’t matter that he has no hope in hell of truly knotting his brother. Sam’s Alpha continues to fight, forcing his premature knot to swell.
Dean spreads his legs instinctively, bows down until his cheek presses against the mattress, ass as far up in the air as he can get it. His arms tucked themselves under his body, between his thighs, and Sam can see his hands clench and unclench as he writhes on the mutt's cock.
Sam didn’t think it was possible with his voice as hoarse as it is from all of the noise he’d been making, but Dean screams as his rim parts around the dog's thick knot.
It forces Dean's hole wider than Sam had thought it could be stretched, and even though it’s all the way inside of him now, Sam can’t believe how much wider Dean's hole is now that the full thickness of the dog’s cock is buried inside of him.
Dean cries, tears dripping steadily from his eyes even as his cock spills onto the mattress.
His hips twitch from his release, and the mutt growls in warning, telling his bitch to keep still and take it. It only takes the light press of the mutt's teeth to his neck before Dean calms down, lets his body go lax and pliant as the dog’s come pumps into him.
It’s a good half hour before the mutt's cock pulls free, come leaking from Dean's hole as steadily as the slick before it, but when it does, the mutt hops off the bed and bends around to lick the fluids off of his flaccid cock while Sammy runs his hands over Dean's still-presented ass.
He can’t help it. Dean’s just... god, Sam can’t even describe it. Used, completely sated after being knotted, but still heat-drunk and wanting. He’s beautiful like this.
Sammy wipes the come off of Dean's dripping scrotum and presses it back into his hole.
He lets his fingers play around in the sticky mess before he puts them to his lips and lickes the slick off of them. Sweet and supple. It’s pure, unadulterated Dean, and Sam can’t help himself.
He unbuttons his pants and pulls them down low on his hips, releasing his hardened cock from its confines. Dean's hips are back to swaying—weakly this time, but Sam can help hold off the worst of the heat for another couple hours.
Alpha come, according to Sam's research, typically acts as a mild sedative. Even as weak as it is with Sam so young, he can help. It wouldn’t have worked for an Omega in the throes of heat, but right now while his heat is coming down from its peak, Sam’s come can help ease it along, give him enough time to stuff his brother with protein, carbs, and electrolytes.
He hooks his fingers inside of Dean's hole and gathers the slick in his hand to lubricate his cock. God, it’s warm. He presses the head inside and strokes himself to completion, letting his come fill his brother's used hole. With any amount of luck, Dean may even be able to get some sleep.
In the corner of the room, the mutt yawns, stretches, and shakes out his coat before trotting back to the bed, cock peeking out of its sheath.
Then again, maybe not.