Down the street a family of four was having fun over on their backyard. Both boys were swimming while Jane, the mother, ate a salad with her husband Carl. Fitness had always been important to the two of them, so even when having a barbecue, they ate very little meat.
James, the eldest decided he was hungry and left the pool to join his mother and father who munched on some lettuce. He couldn't help but to ask when the burgers would be ready, to which the response was soon. Mike, bored decided to follow his brother but as soon as he left the safety of the water Jane and Carl noticed he was sporting a large hard-on.
"I buddy, do you want to go to the bathroom or - " The father was cut short by a grunt, as mike ejaculated inside his swimming trunks. Jane was speechless, Carl rushed to his son's side and James just laughed. Carl started asking Mike if he was feeling well, worried he might be sick but his only response was another grunt as the son started to trust his hips against the air and ejaculating. Carl took a step back, getting up from his kneeling position only to be caught off-guard by his own orgasm. He didn't even notice he was hard, but he soon started to trust in the air like Mike.
James was no longer laughing, he had joined his mother in worrying, both watched in horror as Mike's erection became smaller, while clearly not softening one bit, and Carl's grew in size. But before they could do anything, it stopped. Jane was still sitting at the table when she called to her husband and son, who seem to have a suspicious lack of anything on his crotch, but a certain amount of mass of his chest that wasn't there before. The eldest son approached the pair with caution, but once he thought they were safe, brought both to the table. Jane still hadn't moved from her spot, she was terrible at emergencies, but managed to ask if they were okay. "I don't know what happened, I was there and when I got up I just...It's very swollen honey". He pushed his shorts a bit to see his penis irecognizable and his ball double the usual size.
When Mike did the same, he despaired as it was all gone, he had nothing between his legs anymore. Carl noticed that was not quite right as a very familiar shape could be seen in his now tight and wet swimming trunks. A sense of confusion gripped the family as something the didn't think possible was happening to them. Finally deciding to do something, Jane ran inside to grab her phone, leaving the three men alone.
Mike could not stay quiet, he was pacing around, rubbing his legs together trying to make sense of his new features. Soon, however, he started doing that because it felt good. The more he rubbed his legs together the harder it was to stop. James was pacing around somewhere else and their father was fixated on his engorged member to notice as Mike's chest started leaking milk and gaining weight. His tits ballooned to huge levels in a few minutes while the white liquid when from leaking to flowing. The small Mike was now as tall as his father and even heavier.
Carl's penis had transfixed him for a while, so when he looked to his son after hearing a mix of moans and moos he was shocked. He saw his ears pointed, and a cow tail swaying behind his humongous ass. At first he screamed, catching the attention of James, who also screamed. Seeing his soon with his fingers deep inside his own pussy was a punch to the gut, and he didn't know how to react, but a part of him knew. His penis once again came to life fast, ripping his pants in the process, freeing his member. That's when he realized that it was a bull's cock.
Jane had no luck with the emergency numbers, as the explosion at the zoo kept everyone busy. She was returning to the backyard when she saw the scene. Carl was jerking a penis larger than she had ever seen before while a naked Mike, looking like a fat woman fingered herself. James was also watching, unsure of what to do, but worryingly but mother and son felt a tinge of arousal at the scene, especially James that had a large erection on his tight pants.
Meanwhile, Carl was building more muscle than he had ever had in his life. He felt good and powerful, and free like never in his life. He looked at Mike, now with patches of fur in several parts of his voluptuous body and started to loose himself even more. A tail at the end of his spine and his feet quickly developed into hooves. Mike was also lost in the pleasure, he had already taken notice of the large bull in front of him but had not made the fist move yet. That changed when both bovines saw James grunting and humping the air like they did. Clarity overtook his brother's mind and he tried to walk over to help his brother. Had he noticed however he had hooves now, he wouldn't have fallen on his hands, exposing his drenched pussy to a bull that had not become clear of mind. Mike felt his father mount him before he could move again, and in a second, all worries about his brother vanished, he only cared about being breed like a cow.
James had it rough, not only had he no one to stop him, he had a show in front of him propelling his lust and in exchange, his transformation. His cock was smaller and smaller, sinking inside him while he tried to jerk it on his knees. He felt like he was doomed, his father and brother were half bovine, a black bull and a black and white cow and he showed the same symptoms. However a last ray of hope showed itself as his mother stood by his side. She would stop him and help him avoid the fate that had befallen his family. However once he turned his gaze, he saw something that shouldn't be there, a small-ish bulge on her bikini bottoms that was growing. Since said bikini didn't leave much to the imagination, once it grew just a bit more, said member slipped out and not long after the two large testicles fell through and tore the fragile knot that tied the front and back of the bikini together. Her new balls, produced new hormones and sperm, the later of which flew from his cock and sprayed all over the backyard.
James was fixated on that new appendage, it looked and smelled amazing to his lust filled mind, and Jane couldn't help but notice his face getting closer and closer to it. She knew this was wrong, but she didn't have the will to stop her son. She could feel his heavy breading on her new appendage and it drove her insane. She grabbed his head and shoved her bull cock down his throat.
It was hard to deal with the immediate torrent of cum going down his mouth, but he adjusted surprisingly easy. He saw as his mother gained more muscle, just like his father, only to immediately gain fat on top of it. Her hips widened and her small tits flowed with milk as they grew in size, something he could feel happening with him as well. Two horns started breaking from James forehead, not as big as her husbands, but larger than Mike's and the one's that just started growing in James own head.
Meanwhile, father and now daughter enjoyed each other and the scene in front of them. Their transformations had ended a while ago, but the decided to watch instead of help hasten the transformation, not that they would need it. The scent of bull and cow pheromones in the air made resisting almost impossible.
James tail now stood above a massive ass, and fur had began to spread in his nether regions, similar to his mother. Jena's belly was larger than either of her daughters (former sons), a true testament to her fertility, which she could still make use of as a cow pussy sat behind her cock.
With a third torrent of cum flowing down James throat, both were ready for more. James turned around on his/her own, and Jane jumped at the opportunity. Her cock parted his pussy for the fist time, and both moaned and mooed in pleasure. James's wall immediately clamped on the male member, causing a mutual orgasm to both. Like Carl, dark fur started to overtake Jane, while pure brown covered James. More moos followed as the final parts of the transformation took place, and the muzzle ended at the same time as Jane filled her new daughter once again.
Both Carl and Mikaela (as she would have been known later) approved of the changes, but Jane was still dealing with her male and female hormones and she charged the other fertile female, trying to shove her husband out of the way. Unfortunately she was not as strong as he was, and he restrained her against a wall, showing the she-bull that he was in charge there. Such show of dominance earned the husband an invitation from her to fuck her female half, for which he happily obliged. After a messy affair, with liters of cum and milk being spread over the area, Jane apologized for going crazy. Of course Mikaela forgave her mother the only way she knew how, allowing her to mate with her, while the bull Carl worked on putting a calf in the cow soon to be known as Janice.