TSO Style: Basically someone could buy a pet carrier from buy mode either for dog or cat. Say, they bought a dog carrier. They would open the carrier and randomly a breed of dog would come out. If a golden retriever came out of the carrier then that would be the dog that would forever come out of that specific carrier. The same thing happened for cats. The 'normal' (golden retriever, german shepherd, orange tabby, ect) pets had higher chances of randomly coming out of the carrier while the 'rare' (cheetah, afghan hound, pink poodle, ect) ones obviously had lower chances.
Here's an example of how it looked like. Forget what's written there, this was the only image I could find.
TS Unleashed Style: Sims traveled to a pet store and there could interact with the dog or cat (like shown in the picture from the opening post) and then got to
choose what dog or cat breed they wanted. (Had nothing to do with carriers).
Note: The carrier was bound to the pet, it's not like you had to buy it separately. So, before someone would log off they would put the pet back in its carrier or it would run away other wise. Unless their roommates took care of it.
People bought and sold carriers so people could easily just get the breed they wanted without having to go through the trouble of wasting a bunch of money on carriers that would never give them the type of breed they wanted. Like, when someone went to a pet store lot on TSO there would be rows and rows of pet carriers. In front of the rows there would be a sign saying the carriers on the yellow line are all dalmatians. So, if you wanted a dalmatian you knew to buy the carrier off of the yellow line. These pets stores had so many different kinds of breeds because most of them were trying to find the rare pet and sell it for millions of simoleons. To make back the money they're gambling on trying to find the carrier with a rare pet inside, they sold carriers with normal pets inside to other people.