Sasuke doesn't question it, at first.
Karin's done stranger things before, he reasons. There are a lot of things he wouldn't question immediately, just because he's seen some truly incredible things happen when Karin gets… creative.
Studying under Orochimaru truly did wonders for a person's ability to create new techniques.
But the puppy doesn't disappear for some experiment or training or get sent to a lab on the other side of the country. After a week, Sasuke has no choice but to question it.
"Karin, why isn't that puppy gone yet?" The question is bland, monotonous, really, and he asks it during dinner. He doesn't care for the creature very much, but it's starting to blend into the background environment and that's usually a sign that he's getting too used to things.
"It took you this long to notice?" She scoffs in return, rolling her eyes.
"I noticed, Karin. I just didn't ask."
"Hmph." She smirks at him, spinning a chopstick through her fingers. "I adopted it."
"Why not?" Karin tilts her head to the side, still smirking. "Does it bother you?"
"I prefer cats." It's really the only answer he can think of, since the dog really hasn't been that much of a bother.
"You can get a cat if you want. Just remember that you'll be the one taking care of it." Karin shrugs. "Why, do you want to get rid of the dog? I think it's cute."
"You didn't really ask my opinion on it."
"I don't need your permission for things, Sasuke." Karin's smirk softens into a smile. "Besides, I thought it would be good practice."
"…Practice for…?" Sasuke is very, very confused, and that seems to be something Karin anticipated, since she just rolls her eyes with that same smile.
"For the little one."
"…Little what?" Sasuke knows a few things that phrase could apply to, but apparently, Karin's not going to give him the time to figure it out.
"Dammit, Sasuke, I'm pregnant!" She jabs her chopsticks at him. "You're going to be a dad, moron!"
Sasuke's face doesn't seem to be under his own control, because he's pretty sure he's somewhere between 'Deer in the Headlights' and 'Naruto with an all-you-can-eat coupon for the ramen stand' right now.
"Yes." Karin huffs and crosses her arms. "And the puppy stays."
It takes Sasuke another fifteen minutes to realize he forgot to ask something. "Karin, what's the dog's name?"
"…Promise you won't laugh."
"Karin, when have I ever laughed?"
"…I named it Taka."