The term "teacup dog" is an informal designation given to various breeds of dogs that are particularly small--usually 2 to 4 lbs. The nickname comes from the fact that many of these dogs, especially while they are still puppies, are small enough to fit in a teacup. Many teacup dogs fall under the breeds designated by the American Kennel Club as toy breeds, while others are simply small variations of other breeds.
When many people think of small dogs, they think of the Chihuahua. Chihuahuas are naturally small. Very few grow to more than 6 lbs. Many believe they evolved from breeds of small foxes, many of which are in the highlands of Mexico's state of Chihuahua, from whence the breed received its name. Chihuahuas tend to be patient but alert, and react adversely to cold.
Teacup Yorkies are a subset of a breed more commonly referred to as Yorkshire terriers. The breed hails from the city of Yorkshire in England. Yorkies are typically between 4 and 7 lbs., though by definition "teacup" dogs are generally no more than 4 lbs. fully grown. Yorkies are energetic but family oriented. They like to chase things because of their history as a breed used to chase rats and other small creatures in clothing factories.
Maltese dogs are another breed that is naturally small--generally between 4 and 7 lbs. The breed originated in the ancient trading port of Malta in the Mediterranean. Maltese are among the most well behaved of small sized breeds. They also tend to be fearless and full of energy.
The playful pug is one of the oldest breeds of dogs in the world, existing well before 400 B.C. in east Asian areas such as China. Pugs usually weigh between 14 and 18 lbs, but "teacup" pugs are typically much smaller. These small qualities can have harmful health effects for the dogs.
The Shih Tzu is another small dog bred even smaller for the "teacup" market. The breed itself is more than 1,000 years old and comes from ancient China where it was prized by royalty. Similar to pugs, the Shih Tzu usually weighs between 9 and 16 lbs. when fully grown, but is bred much smaller for the "teacup" market. Shih Tzus are usually energetic and alert, but also friendly and outgoing.