(slightly ret-conning so that it is a dog collar, not a cat collar)
You battle with your conflicting instincts. You've never felt so divided, so powerfully compelled both to put the collar on and to put it on your girlfriend, but eventually you just snap and before you know it you've fastened it around your neck.
You'd been worried before, now suddenly everything feels right as a wave of complacent satisfaction washes over you. Brooke smirks at you with a cockiness you'd never seen her display before. "You seem to like it. Good. It suits you." The sexual implications in her voice are hard not to notice and your cock twitches.
Suddenly you do feel a little self conscious though, wearing something so kinky in public. "Uh, maybe I should take it off until we get home though..." you say.
"No." Her voice is firm and commanding. "Keep it on. I want everyone to know you're mine."
You can't help but feel submissive, and aroused, before this new side of your girlfriend, but still a little whine escapes your lips. "Brooke, it's embarrassing..."
"Keep it on, and I promise you won't regret it later tonight." You don't like it, despite the innuendo, but you comply.
As you go to check out, you can't help but find it strange the way the cashier acts. He never addresses you once, instead talking only to your girlfriend. In fact you notice that none of the people there seem to address you, or even look you in the eye. You don't say anything about it while in the shop, but as you leave you mention it to Brooke.
"Oh I'm sure it's nothing, dear." she says. "Don't worry yourself about such things." Then you arrive at the car. Without even thinking about it, you find yourself heading for the passenger seat while she takes the driver seat. But wait, wasn't this your car? When did she even get the keys? The more you think of it, the more confused you feel, until eventually you're questioning if this was even your car at all.
"You think to much." you hear her say. "and it's clearly got you all upset. Just let me think for you, ok?"
You can't tell whether this new persona of hers is part of a role play or not, but you find it surprisingly easy to take her advice, and the rest of the ride home is peaceful.
Finally you arrive at the house. As soon as she closes the door behind her she grins seductively at you. "Now, clothes off. Pets don't wear clothes."
You grin back and comply, hoping for some kinky sex. In moments you're naked and on your knees, your hands playfully in a dog-like begging position. You give two enthusiastic barks. Just a playful joke, yet you're impressed by how good your bark impression was.
"Yes, good boy." she says with a smile as she unbuttons her blouse to reveal her lingerie bra, a DD cup that still struggles to contain her assets. Your girlfriend had always been busty, and you were certainly a boob guy. You bark again all the more enthusiastically. She slips her skirt off as well, leaving herself only in her underwear, and soon you're following her magnificent ass to the bedroom, on all fours of course.
You hop on the bed, but she scolds you. "Bad dog! No doggies on the bed."
You smile playfully whine in response, and then fall over on your back as if asking for a belly rub, but with your cock sticking out straight and tall it's a rather lewd sight. This role play stuff was fun.
She smiles. "Alright, just this once" She seductively crawls onto the bed, and you know if you had a tail you'd be wagging it. "Puppy wants a rub, huh?" she says, and she does rub your belly a little, but her hand quickly moves down towards your dick. As she works your cock, the feeling is absolutely incredible, and yet you feel more submissive than ever. Your whole body begins to tingle, but you scarcely notice how unusual it is, so lost are you in the sexual pleasure and this new pleasure of submission. Eventually you do notice a smirk on her face though.
"Ah, so that's how it works." she says.
"mmm...what?" you manage to say between pleasure groans
"Well, it seems that sexual pleasure is triggering your transformation."
"Huh...transformation?" you say?
"Into my pet, of course. I'll confess that it wasn't actually a coincidence that you found that collar. I paid the shopkeep to enchant it to attract you. Of course he warned me that if your will was strong enough you might put it on me, but I knew your true submissive nature was just aching to come out. You were a lousy boyfriend outside of the sex. You'll make a much better pet, and of course I'll make good use of your money and possessions to. You're going to be a beautiful dog-person, dear, a german shepard to be specific."
"What? No!" you say, and looking down at yourself you notice that you've begun to sprout fur! You try to wiggle away from her but she quickly deposits her shapely rear on top of your chest, and without ever having let go of your cock she resumes stroking it.
"Stay!" She commands, and suddenly you find yourself unable to move, bound by the power of the collar. "Now don't be a bad doggie." she says, "Just lie back and let mommy complete your transformation." You want to resist, but the pleasure is even greater than before! You struggle a little bit, but you're hypnotized by it, and before long your only resistance is a desperate and increasingly dog-like whine.
"ssh ssh ssh," she says, "Don't whine. You're going to love being my pet."
as she strokes you feel your body continue to change. Fur spreading, your hands start to grow claws, and your ears shifting up to the top of your head, but as much as you want to fight, it just feels too good!
"mmm good boy!" says Brooke as she works your shaft. "Nice and hard for me. Just focus on the pleasure, and let all your defiant, human thoughts slip away."
It doesn't take long before you give in to the magic and pleasure completely, and start to feel your tailbone digging into the bed, wiggling back and forth in your happiness. You could tell your mouth had formed into a muzzle, and your ears were fully floppy dog ears now, but while some part of you kept insisting that this was bad, you couldn't remember why. Certainly it seemed to make Mistress happy, so how could it be bad?
"Oh you're almost there dear! But we'll need you to cum to make it permanent. So cum!" She said it like it was a dog command., "Come on boy! Cum for mommy! Cum!"
The power of her will crushes any of yours that might have remained, and you cum, hard. "Yeeeeeees" she cries "Cum all that humanity away and become my pet!" You barely register the meaning of her words, though. You just lay there happily giving her every drop of cum in your balls. When you come down a bit from the high you do wonder about her words. Humanity? How could you have humanity? You were a dog. But you see Mistresses's big smile and you forget about it. Mistress was happy! Yay! You must have been a good boy. Maybe she'll reward you with treats or more play time. She scratches behind your ears and your tongue lolls out of your mouth in happiness.
She removes the collar. "Now that the change is cemented, you don't need this and I can use it again. Maybe I'll turn your asshole friend Mike into a nice bitch for you?"
You only tilt your head quizzically. You could understand human speech, of course, but it didn't always make sense. Dog-people weren't as smart as humans.
Then she moves off of you, pulls down her panties, and spreads her legs. "Now come on over here, boy. Mommy almost had an orgasm changing you, but not quite. Let's put that new long tongue of yours to work." You wag your tail happily and rush over to bury your head between Mistress's legs. Life was good.