Rick had always a bit of a size king, so obviously he picked the largest one he could find. He briefly considered the various bestial options. Some of the options were too strangely-shaped for his taste, but there was one that stood out as looking pretty human-like while still being large: the one labeled "dragon."
It was a deep black, though Rick could see hints of red within. It was well over a foot long, and his hand couldn't fully encircle the width. He dropped his pants and situated the phallus to get a better idea of how far down it would hang. He must've brought it too close, because he felt a strange snap and a flood sensations from the penis.
More distressingly, he felt the blood drain from his body, filling and expanding the newly-attached member. Where it had started shy of 2 feet, it rapidly lengthened, slapping onto the floor and continuing to grow. The girth quickly surpassed either of Rick's legs.
Rick started to move forward with his new penis dragging behind, trying to reach the mages, but he doubled over in pain. His hips cracked, and he could see deep-red scales forming at the base of his penis. His muscles strained and bulged as the scales spread over them. His feet lengthened, but the flesh of his toes receded to create claw-like talons. As the scales spread to his rear and along the base of his spine, a strange fold of flesh appeared. It felt as though something was tearing his spine out, but it just kept pulling further. Rick could feel a strange, new weight to his body as a reptilian tail formed.
The scales spread to just beneath his ribs, and his lower body grew disproportionate to the unaffected portion of his body. He could feel a strange growth within his body, like he had just grown a second pair of kidneys. The tail continued to expand, easily 10 feet in length. Laying flat on the floor, his rear towered above him. His new lower body was close to 4 feet tall, without standing.
The changes began to slow, but as a final touch, Rick's hips cracked, and he realized that his hind quarters were meant to be quadrupedal. He felt claustrophobic in this aisleway so he slowly made his way back to the front desk. Hopefully, the mages would know what to do.