You'd have been wagging your tail already, if you could, when you were proudly marking the "generic mixed breed" option. It couldn't be more perfect! You could have asked to be some kind of pure breed if you'd wanted: the perfect pet for the ambitious, status-conscious people you've been surrounded by your whole life. Apparently, whoever was making these serums had genetic samples from some really in-demand breeds. If nothing else, you would have been valuable and in some sense inherently wealthy, at least for the highly sought after genetic material in your transformed gonads. Screw that! Even a mutt is a good enough pet for somebody somewhere, so the life of a mutt is good enough for you.
"Now as a last step before your injection, I must inform you that this is your final chance to refuse this procedure or request a different species," said the doctor once she had you in the injection room with the syringe in her hand. "If you're uncertain about your choice, you can ask to be back to the reception area for more reading material, I can try to answer your questions here, or we can rewatch the introductory video about what you can and cannot expect to gain from species reassignment. If you're simply disillusioned with your present life, you are free to request counseling as an alternative option, or as a final step in the process. You will be bound to your new species (and breed or lack thereof) after the needle is inserted, but not before."
You can translate the doctor's little canned speech easily enough in your head: "Really? a mixed breed? Not a tiger or a falcon or at least a poodle? Where's your ambition?!" Well, screw ambition! This IS your ambition!
"I'm sure," you answer patiently. The doctor smiles and squirts a little of the serum in the air out of the syringe. She smiles and says, "I think you'll be the first mongrel I've made since I started here," seemingly waiting for you to tell her there's been some sort of mistake. You simply smile back and say nothing.
You barely feel the needle going in, but you sure do feel the heat after it does so. You open your mouth and let your long, pink tongue tumble out. Man, it sure got big already! A bit of spittle drops in your lap from the broad, fluttering outstretched half of your tongue. Your nostrils get wet, and you suddenly get a very strong impulse to sniff the air. You can smell dozens, or hundreds of people who have been in this room, and animals, too! (Mainly smaller animals that would be able to change in a little room like this.) The bridge of your nose gets longer, wider and taller, until soon it's taken your mouth with it to go point out in front of you, while your tongue gets even longer to compensate. The prickly feeling of firm hairs sprouting out from the sides of your muzzle quickly fades into the pleasant feeling of whiskers twitching in the air. You feel your ears flop over and practice perking them up and down a little. Your vision blurs up a tad, but it's more than compensated for by your ability to hear things like the doctor's heartbeat and the sounds of people and animals outside the room, even outside the facility.
The doctor chuckles and gives you your very first scratch behind the ears, causing you to groan and lean in, her professionalism disarmed a bit by the sight of a cute animal taking shape in front of her. You're going to love being cute! Old people will make comment on how friendly you are and children will run up to you screaming "doggy". So what if some people look down on you and say you're "just an old mutt"? If anything, it will be a relief to be "just" something you're happy with, rather than constantly trying to think of something or someone else you'd rather be or try to figure out what everyone else wants you to be.
The feeling of a hand on your fur makes you perk up and realize your fur has grown in, or at least a lot of it has, anyway. The doctor is petting your back, which has lots of bushy brown-and-black fur. Something about your torso reminds you of a German shepherd, but your gently domed head and pointy, floppy-eared face are more retriever-like. You shift uncomfortably in your seat as your legs start to change. You reach for a thinning, lengthening foot and feel a set of rough pads forming on the bottom. So much for needing fancy shoes to go to a prom, a wedding, or a business meeting to make a good impression in case someone wants to promote you. You'll be able to run through the grass barefoot and leave your owner in the dust during a game of fetch. What would Coach Burman say about that?
Something hairy starts to twitch a little over the top of your butt, and you turn around just in time to see a hairy nub grow longer and a bit bushier behind you. By the time it's six inches long, you have at least some say in what direction it goes and how fast. By the time it's only a bit shorter than your back, it's obviously getting a slight curve at the end and still growing. You start to feel quite a bit of recoil in your hips from wagging it. You let out a playful bark through your new muzzle. You think back to your bitchy speech teacher who gave you a D back in college. She said you needed a tremendous amount work on your pacing, diction, and articulation if you had any hope of running a business or winning an election someday. What would she say if she found you'd given up your human voice altogether in order to bark at things and wave around a big furry plaything on your ass to get your point across about how you were feeling? So much for being some sort of mayor or something!
You look forward again to watch your hands change shape to maybe help you keep your balance better in your seated position on your canine haunches. Your fingers are getting way too short to do much with, but you'll be able to dig in the dirt nicely with those new claws of yours! Your paws start to feel nice and cushy when the pads puff up like the did on your toes. Watching your little dewclaws take their final place up on your wrists--where your useful opposable thumbs used to used to work as hard as you did--reminds you that you won't be doing much for yourself from on. You'll live your life on all fours, padding along to wherever a human decides you should get to go. Maybe you'll be ambitious enough to drag them along from the end of your leash!