This is just a quick story I wrote on my phones notepad because I wanted to get this idea out. Luna is of legal age in this story. This is an Au of course.
All characters and settings belong to J.K. Rowling. The idea clearly doesn't.
This story contains bestiality with an animagus, if you don't like turn back now for your own good!
Everything is silent, blanketed in darkness, the full moon's soft glow and the gentle twinkle of the stars the only guiding light. A barely audible pitter patter of sock clad feet against the stone floor made the dog's ears perk up, he stalked quietly toward the sound. His dark coat hid him in the shadows as he approached. The dog stopped when he caught sight of the source of the noise, standing by the window in the cold stone corridor was a lone female figure. As if she knew he was there she turned her head, icy blue eyes staring right at him, her expression dreamy and seemingly unfocused, she smiled and gracefully turned to face him. His eyes widened at what greeted him, under her black cloak was nothing but a pair of mismatched stockings. He didn't know what to do, he was frozen to his spot. Sure he was quite a ladies man, but this girl was unlike anything he was used her empty basket on her shoulder she moved toward him, her small breasts bouncing with each step.
"Hello, Professor Black." Her voice light and airy. Now standing in front of him, she knelt to his level and started scratching his ears, and petting his face. Her scent tickled his nose, she smelled softly of peonies. Before Sirius could stop himself he licked her, his tongue swiping against her pink lips. She gasped softly then giggled.
"What was that for, Professor?" He didn't answer and licked her lips again, this time intentionally, as he hoped to sneak his tongue past her sweet tasting lips.
Luna giggled again, he didn't stop licking her as she did so, she opened her mouth to say something to him but he stuck his tongue in it before she could make a sound. He licked at her sugary cavern a few times before he tried to pull away from her. Her lips closed around his tongue, sucking on it as it retreated. Slender arms wrapped around his thick neck and her pale naked breasts pressed against him.
"That was nice." She said softly hugging the large dog, enjoying the feeling of his slightly wired fur rubbing against her bare skin.
He sniffed the air, checking to see if anyone was close by, when he smelled none he pulled away from her and started walking down the hall, he pause and turned toward her to signal her to follow. Luna did so without hesitation, her plan to pick moonflowers postponed until her curiosities were satisfied.
The dog led her out of the castle and into the forbidden forest. They didn't go far, just enough to where no one would accidentally stumble upon them. She set her basket on the ground and looked around the small clearing, a large rock bathed in moonlight is what caught her eye, she walked to it then draped herself gracefully across it. He followed her with his eyes taking in all her seemingly otherworldly beauty, her flaxen hair pooling around her, her pink lips curved into a soft smile. Her legs were spread and he could see her soft, pink folds. Sirius cautiously stalked between her slender legs, taking in the scent of her pussy. He pressed his cold nose into her well trimmed curls, sniffing her once before he let his long canine tongue lap at her, starting from her asshole all the way up to her clit. Luna whimpered at the feeling, opening her legs wider welcoming him to taste more. He moved his focus to her clit, his lapping becoming much more enthusiastic and slobbery now, an animal instinct taking over and fogging his brain in a way he had never experienced. Luna was now moaning and thrashing her head back and forth, his name escaped her lips a few times but he could barely comprehend it, the only thing his mind understood was this fierce animalistic need and the honeyed taste of her juices. It didn't take long before she was starting to grow louder in her exclamations of pleasure, signaling she was close, a few more licks from his broad tongue and she sat up to clutch his scruff and hold his snout and tongue to her cunt as she rode out her orgasm. Luna fell back against the rock, Sirius lapped her juices up before he licked his chops and mounted her, his swollen red rocket jutting proudly from its sheath. She was still not out of her post orgasmic daze when she felt something hot, sticky and hard start poking at her thatch of curls. She shakily sat up on her elbows she looked down and saw him trying to guide his large red cock into her slick cavern, but all he was accomplishing was getting her pubes covered in his pre cum. Luna sat up and put her hand to his chest, he growled but she pushed him away from her before she got off the rock, she slipped her cloak off and set it on the rock before she sank to her hands and knees. She crawled to him and then under him, he felt her soft delicate hands wrap around his girth, stroking him carefully. He then felt her petal soft lips wrap around his pointed tip. Her tongue lapped at his hot flesh as she took him into her mouth, she got all but his knot in, his tip brushed the back of her throat as she started to bob her head. He wrapped his paws around her waist, mounting her face. He began to thrust his hips, setting the pace faster than she had, thoroughly fucking her face. He started licking her spine, causing her to shiver, she could hear his breath quickening with his thrusts, he began humping her throat faster, instinctively trying to force his knot in as he began to spurt his hot jizz into her hungry little mouth. Luna gulped as much of his thick cum down as she could but there was too much and a lot of it spilled down her chin and neck to her small cleavage. She pulled away and stared dreamily up at the panting dog who stared in awe at the strange young woman before him. She smiled her faraway smile and still on her hands and knees turning her shapely ass toward him. He stalked toward her, his spent cock swelling up once again as he sniffed and licked her pussy, taking one last taste of her pussy before he mounted her, poking around until he felt her slick heat around his tip, his hips snapped forward stretching her tight pussy open. Luna gasped and whimpers in pain, the smell of blood invading his nose. She shook as she sobbed, but she managed to look over her shoulder at him with tear filled eyes.
"Don't stop Professor, I'll be okay." She whimpered, still managing the faraway tone. He couldn't help himself and began thrusting into her rapidly unable to control his animal lust. Luna was quiet for awhile, until the tip of his big doggy cock reached her womb. The moan that escaped the girl made his cock throb inside its new confines, his front legs wrapped around her waist as her leaned his upper body on her delicate back, his snout pressing against the back of her neck. He started to lick lovingly at her skin, pounding her pussy mercilessly. Luna's arms gave out and she collapsed her face now cradled by her arms as she panted, whimpered and begged him to keep fucking her. She nearly screamed as she was hit suddenly by the most intense orgasm she'd ever experienced, her pussy clamped down on his cock and he pressed into her trying to force his knot into her freshly devirginised pussy. She felt his knot and dazedly tried to get away, but he wouldn't let her, he growled and bit the back of her neck and shoved his knot into her little opening. She screamed loudly, so loud it echoed through the forest. As soon as his knot was in her he started to cum, shooting thick gooey cum straight into her womb, where the tip of his dog cock was nestled passed the opening. He panted, his teeth still holding her neck, his knot locking them together. Luna was sobbing softly, he could feel her back shaking against his chest, he let her neck go started licking it. She turned her head so she could look up at him from the corner of her eye, he leans down and licked the tears off her cheek. She smiled weakly up at him then slowly passed out.