Mary moaned in relief as Rob continued emptying her udder. He had taken a minute's break to grab a few large mixing bowls from the kitchen, and was squeezing two teats at a time into different bowls. The once impossibly massive udder was now down to a better size; less than two feet across and shrinking.
Lost in the sensation of the milking, Mary took little notice of more feelings occurring through her body. Her rear end ached, and a pressure was mounting on her forehead. She idly scratched an itchy ear as she caressed her udder with the other hand.
Rob looked up from his milking after Mary gave a particularly loud moan of satisfaction. He was shocked to see a small pair of horns growing out of her hair. They lengthened slowly as he continued squeezing her udder's teats, but he dared not stop milking Mary for fear that her udder would start growing again. He kept stealing glances back at Mary's face, watching the progress of the horns as they curled upward on either side of her head. Her ears seemed to be getting bigger, too. Now they were emerging from her head of long hair and started to develop points. Mary seemed to be blissfully unaware of these new changes. Her eyes were shut and she continued to rub her udder, which by now was approaching one foot wide.
He stopped once all of the bowls were full, and began carting them over to the sink one by one, dumping them out. When he resumed the milking, Mary began to come out of her trance. She watched him with a blushed face as he finished emptying her udder. Rob felt a little better after seeing Mary regain her senses. "Is that better?" he asked once her udder was as small as a volleyball.
"Yes, much better," she sighed. "Thank you. This must be so weird for you."
"It's not weird for you?" he asked incredulously.
Mary laughed, blushing again. "Yeah, it is. I never expected the Vial to do that to me..."
"Vial?" Rob wondered aloud. Mary shifted her eyes back and forth, probably trying to decide whether or not to tell him what was going on.
"It's a very odd story," she began. Mary recounted her experience with the Cow vial earlier that day. She finished by saying "...and I don't know why I re-grew this udder. I didn't take any more of the Cow vial."
"That's not all you've grown," Rob said, pointing to her head. Mary felt her ears, which had lengthened so much they were flopping over. They were covered in white fur, and her left ear had a black tip. Mary gasped, then felt the horns growing above them. She darted--bathrobe loose and with her udder flapping against her legs--to the bathroom. Rob joined her.
"But I didn't have these before," Mary said. "I only had a lower half like a cow." She reached behind her, inside her robe. "There's something else...yeah, I've got my tail back." Rob saw her hand emerge from the robe holding a ropey cow's tail, complete with black spots and a blonde cuff. "But on the bright side, at least my udder isn't growing again."
"A side effect, maybe?" Rob wondered. He still couldn't believe there were potions that could change people into whatever animal they chose, but if there were, there were probably plenty of risks involved.
"I don't know," Mary said. "All the note said was to take an entire Vial for full effect. It didn't say what would happen if I only took part of one."
As Mary closed up her robe, pulling the string tight to restrain her udder, Rob wandered over to the box Mary told him contained the Vials. He was about to open the box when the doorbell rang.