Rob, a bipedal straw-yellow tabby, walked over to the box of vials to peruse what other options there were. He rubbed his fully belly, which bulged out slightly from all the milk he'd drunk. He wanted more, but at the same time, felt very full.
His furred paw brushed along the sides of the vials as he read what they'd do. He stopped at one vial. Intrigued, curious, and just a bit uneasy. A strange thought had suddenly occurred to him. If he used this vial, he could drink so much more of the milk. His mouth watered and his tongue came out to lick his nose. Thoughts of milk, Mary's luscious warm milk going down his throat soon started to become overpowering. He yearned for it. It was all he could think about.
Any warnings or second thoughts were flittered away as Rob, trembling, took the vial out and gulped all of it down. Mary hadn't been watching, she was engrossed in the sensations of her large and full udder pressing against her legs.
Rob felt a shiver as the vial took effect. Beyond curious, Rob watched as his soft yellow fur receded into him and his flesh first turned dull, then shiny. It blossomed into bright yellow rubber. The shiver of flesh turning to rubber cascaded down and up his body. His tail quivered and quickly became rigid and bouncy. He bobbed his hips back and forth and felt his tail remain in shape, but thwap against his rubber legs.
In no time, Rob had become a yellow rubber cat. He squeezed his arm, watching it deform and then snap back into shape. He was completely hollow, except for some milk in his feet, which swirled around and tickled a bit. Now he was ready to suck all the milk he'd ever want from Mary...