Jeff raced back downstairs to the Vial box - he could hear Mary following slowly, her huge frame causing the stairs to groan and squeak in protest.
"I want to get outside before I'm unable to fit through the door!" she growled as she lumbered through the living room on all fours. Jeff was shocked to see that her hands had changed into enormous paws, each the size of dinner plates to match her hind paws. As she disappeared into the kitchen, he noticed a small, furred nub of a tail had emerged above her hugely-rounded rear end and was wiggling slightly as she walked.
Turning back to the Vials, he perused his options: he wanted to be something large, like Mary, but not another bear... apparently the liquid was truly magic - now was the time to experiment! He dismissed the elephant vial, as well as the rhino, hippo and aquatic options before stumbling onto a section marked "Mythical." There were vials for merpeople, centaurs, fairies and ... Ah-Ha! A Minotaur! Jeff snatched the vial up and quickly gulped down the contents before racing for the backdoor Mary had left open.
He found his wife standing with her back against a large oak tree in the corner of their yard, scratching her hide against the rough bark and growling in pleasure. Her mouth had pushed out into a long muzzle, and there was almost nothing left to distinguish that she had ever been human - it really looked as if one of the largest bears on the planet had wandered into their backyard! Walking closer, he realized she had to be almost 10 feet tall, standing on her hind legs as she was! Her neck was almost as wide as her shoulders, while her arms were probably as big around as her torso had been!
"Incredible," Jeff gaped, in awe of the creature his wife had become. A sudden tingling in his head signaled the start of his own changes, and as he reached a hand up to his face, it was rapidly pushed back as his nose and mouth broadened and expanded. He could see down the boxy slope of his own new muzzle, and as he watched, the tips of two furry ears quickly appeared in his peripheral vision! An intense pressure built just behind either temple, quickly followed by relief and the sensation of increasing weight, and he could only guess that his horns were growing out of his head; he wondered how long they'd get, or if the curved upwards or back?
Looking down at himself over his bovine nose, he could see that his body remained human-looking and wondered how much of the rest of him would change - minotaurs were mythical, so who could really say what form the magic would shift him into? The tingling raced down his neck, and he could feel the strength of new stabilizer muscles helping him to support his changed head and weighty horns. A visible ripple descended his naked form, causing his muscles to become more defined even as they expanded and grew. His pectorals tripled in size under expanded shoulders, and his lower torso stretched to accommodate rippling washboard abdominals.
As the tingling centered on his groin, he watched in amazement as his penis engorged, growing to monstrous proportions and standing fully erect up against his abs. Thick, dark hair exploded outward, obscuring his darkened scrotum and rippling outward to encase his thickening thighs before heading lower. As the fur reached his knees, Jeff was pitched forward and barely caught himself on his hands - his feet were stretching, his heels pulling back even as some toes shifted and melted away, the remaining middle toes thickening and growing into cloven hooves. Looking back at his hands, he watched raptly as four fingers became two, and hardened nails encased each digit. Something brushed against the fur on the back of his left thigh, and he swung his huge head around to find a long, ropy tail covered in fine brown hair swinging above his thickly-furred buttocks. A tuft of coarser, dark-brown fur encased the tip, matching the shaggy fur that covered his lower half.
As he pushed to a crouch on his reformed legs, he heard a snuffling sound growing nearer; Mary had finished her back scratch and was approaching on all fours. Jeff knew she was probably about five-feet-tall at the shoulders, and was shocked to realize that he could see over her back even in a crouch. Rising slowly, Mary huffed and mimicked his movement, shoving her bulk up to standing once more. As they rose, Jeff was elated to realize he had chosen perfectly: his new minotaur form matched the animal size and strength of his wife exactly!