They don't live very long. They are HORRIBLY bred. They DO NOT EXIST and most of them either come with, or develop serious health issues very early on in life, or sometimes during their life.
Teacup is just a name/marketing scheme used to scam people into buying an abnormally small dog. That's all teacups are, "abnormally small dogs" because they are bred to be abnormally small, they are prone to tons of health issues, and body/physical and mental malformations. Only backyard breeders use the term "Teacup" and only a backyard breeder would breed a dog, so abnormally small, it's unhealthy. These sickly dogs are often times sold at exorbitant prices, but what people don't know, is that they'll be spending more, just to keep the dog alive. Teacups are created by breeding the smallest "runt" of a litter to another or the same of the litter, often times inbreeding is used because it seals in the "small trait". The horrid way these dogs are bred, coupled w/ other things, make them horrid dogs to get.