People have had pet rats for a long time. It just wasn’t such a publicized thing. Then people let go of the stigma of Rats, and started loving them as pets. As early as the 18th century people had rats for pets, however many of them involved them in cruel games with a terrier. Hence Rat Terriers.

I have personally had 3 rats. They are very clean, as long as you keep their cage clean. They can be easily trained, and a lot of the time they like to cuddle.