There are a couple of important differences between toy and teacup dogs, the most significant of which usually has to do with breeding. In most cases, dogs in what are known as “toy” breeds are naturally small is size and, as their name suggests, are often reminiscent of stuffed animals or other related toys. In general these sorts of dogs are fully healthy and active, just petite by nature. Teacups, on the other hand, are usually bred specifically to be as tiny as possible. No breeds are normally naturally as small as many teacups turn out to be, and most have a number of often serious health complications as a result. The majority of breeding associations and kennel clubs around the world do not recognize any teacup breeds, and many actually look down on the practice of size-selecting and other genetic manipulations.
The term "toy" is used to describe certain breeds or groups within breeds that are very small in size, typically under 12 inches (30.48 cm) when fully grown. Pugs, Maltese, and Chihuahuas are just a few examples. "Teacup" is not an official term, and is often used to describe any dog that is very small — often unusually small — in size.
There is no official size standard for a teacup dog, although the name comes from the idea that the animal is small enough to fit inside a teacup. The name is generally considered to be a marketing term, and none of the major international dog registries recognize the group. Toy and teacup dogs may share that smallness of size, but puppies sold as teacups are often those that were born prematurely or those that were bred from two exceptionally small parents.
It’s almost universally true that a teacup dog’s small size is the result of genetic manipulation, particularly in breeds that aren’t normally very small at all. In most cases, breeders select abnormally little dogs to breed with each other, and usually also watch out for mutations leading towards stunted growth that can be manipulated.
Some backyard breeders and puppy mills breed teacups for the sole purpose of producing the smallest dog possible for the simple fact that they assume tiny dogs will fetch higher prices. This does often seem to be the case, particularly among clientele who want to use the dogs as something of an accessory. Families and individuals may also be willing to pay a bit more for teacups because they look cute or seem like something of a novelty.
Most toy breeds, by contrast, are small simply by their nature. Humans don’t need to intervene or be involved at all for these dogs to be little even when they’re full sized, and breeding isn’t usually selective — at least not where things like height and weight are concerned.
Health and General Wellness
Another main difference between toy and teacup dogs is their general health. Toy dog breeds are typically healthy while teacup dog breeds tend not to be. Teacup dogs usually are smaller and frailer than toy dogs, and they characteristically contract illnesses and diseases faster, have more medical complications, and are more apt to die younger. In addition, teacups often have extremely delicate bones and respiratory problems.
Research and Authentication
Anyone interested in the “official” specifications of any breed or size demarcation can often find a lot of information from national registries, breeding associations, or kennel clubs. Toy dogs are usually recognized all around the world, whereas teacups typically aren’t. The American Kennel Club (AKC) in the U.S. has a compilation of all the registered breed types on its website. The site can serve as a good reference tool to examine and compare various dog breeds, even for people living or searching for dogs elsewhere. In the U.S., most qualified breeders raise dogs according to the AKC's standards, usually by mating two champions to preserve the integrity of the breed. In all cases, the lineage and parentage is documented, which usually makes genetic manipulation pretty obvious.