Try to collar the storekeeper.

“Hey,” you call, “can you help?”

The storekeeper approaches. “Yeah?” He asks. You move to put it around his neck. “Hey! What the hell?!” He cries as it flips around him.

“Catgirl...” you say. Nothing happens.

The storekeeper reaches up and removes the collar. “You really should have read the instructions... “ he observes then wraps it about your kneck, muttering a word. You feel it lock and your body tingles. “Nympho, submissive, constantly in heat, sexy as all hell, utterly obedient miniature dog girl.” He growls.

You let out a gasp of surprise as you start to shrink, your hips widening, chest swelling with rows of breasts, ears shifting, mouth changing, fingers and feet shifting and tail growing. You whimper as your body conforms to masters command. The word nympho rings in your mind as the mental changes assert themselves. “Please. It was a joke!” You wine.