The Pet Store Of Course

"We're going to the pet store, of course, to get you everything you'll need." You say, as if it should be obvious.
"What things?" Amy asks meekly. You snicker.
"Oh, that will be a surprise, but I'm sure you'll enjoy it."

You arrive at the largest pet store in town, full of the noises of: barking, mewling, chirping, tittering and various other sounds. That's coming from all the animals. Another section of the store is devoted to animal accessories and toys, it is to this section you make a beeline. You're intercepted by a customer service representative in a blue shirt with a yellow paw-print on the breast.
"Hi," she says. "I'm Jenny, can I help you with anything? Are you looking for anything specific today?"
"As a matter of fact, we are." You reply, "I recently acquired a new pet, quite a large animal, and I'm here to buy absolutely everything I need to get it set up in its new home."
"Excellent," Jenny says, her face breaking into a smile. "Shall we start with the basics? You're going to need some way to transport it, we have an excellent selection of cages." You follow Jenny to the cages. The thought of caging Amy gives you a raging stiffy.

You spend the rest of the day in the pet store, leaving with thousands of dollars worth of things. You splurged, and bought the best of everything. You want to be the best master possible for your new petgirl. Amy is silent the entire ride home, you assume she's still just digesting things. However, there's one more surprise in store for her. On the way home, you stop at the local sex shop. Unbeknownst to Amy you've actually been here a few times before, discretely. You crack a window and leave Amy in the car, while you go into the store.
"Please, Elliot, take me with you." She cries after you, but you ignore her.
You return, twenty-five minutes later clutching a plain opaque bag.
"What's in there?" Amy asks.
"It's a surprise," you reply. "You'll find out soon enough."

You arrive home and unload all the pet gear. There is a lot of it, and it will probably take you a while to get it all set up, but the essentials are ready to go. Now you reveal what you bought at the sex shop.

Amy looks shocked. "It's a butt-plug cat tail." You explain. "Isn't it great. You'll be a proper pet now, with a tail and everything."
"Do...do I really have to wear that?" Amy asks, tremulously.
"Hey, you said you'd do whatever it takes, right?" You ask, accusingly. "Well, this is part of my fantasy." She just stares at the thing, with a look of horror on her face.
"It's so big." She says softly.
"Yeah, I got the biggest one they had, got to make sure it's in there nice and tight, and if it stretches your asshole a bit, so what?" You reply.

It takes her almost half an hour to work up the courage to insert the butt-plug, but with it in, and the accompanying cat-ears clipped into her hair, she truly does look like a beautiful pet-girl. There's only one thing left.
"It's time for the one thing every pet needs." You say seriously. "A collar." You produce the one you'd picked up at the pet store.

It's very plain, but that's part of your plan. As Amy sinks deeper into submission and becomes a better pet, she will get a more elaborate collar.
"Now, take off your clothes." You command.
"What, really? All of them?" Amy asks.
"Of course. Pets don't wear clothes do they?" You ask rhetorically. Your girlfriend slowly starts to strip. She's shy and nervous, despite the fact you've seen her naked before. It's cute.

"Kneel." You command, when Amy is completely naked. Slowly, hesitantly, she complies, dropping her arms and letting her breasts hang free. Your formerly innocent, adorable, cute girlfriend is now naked on your floor wearing a butt-plug and cat ears. Your wildest dream is rapidly becoming reality.
"I want you to swear to me that you will be the best pet you can possibly be. I want you to acknowledge that you submit to me, as your master, and rely on me to care for you. You relinquish your right to be human, you are a pet-girl now. Once you do that, I'll attach this collar, and our new relationship can begin.
Amy looks up at you with those big blue eyes and says.
"I vow to be the best pet possible. I relinquish my humanity and my rights to my new master Elliot, and trust to him to love me and care for me."
Reverently, you fasten the collar around her neck and secure it with a little SNAP. Your fantasy has now become a reality. You have your own pet-girl.