Chihuahuas are the smallest of the dog breeds. They originate from the Mexican state of Chihuahua. The main differences that exist in the Chihuahua breed are color, length of fur and body type. One of the most noticeable differences between these dogs is that some have apple-shaped heads and others have deer-like heads.
Chihuahua's with apple heads have a short nose and muzzle. The top of their skulls are rounded and more prominent than the rest of their skull. As the top of the skull rounds over, it sinks in right above the eyes. This sunken area is called a "molera"; it is used to differentiate between apple and deer head Chihuahuas. The presence of a molera can be determined when the dogs are still puppies.
Deer hear chihuahuas do not have a molera. Their skull is uniformly shaped and the muzzle is in proportion to the rest of the head. Their noses are not as small and flat. If a Chihuahua does not have the apple head characteristics, then it is a deer head Chihuahua.
Deer head Chihuahua's tend to have less Chihuahua specific respiratory problems. Tracheal collapse in particular is a major health concern with Chihuahuas. Deer heads suffer from it less often because their tracheae are not as small and vulnerable as apple heads. Also, the longer muzzle gives the small dog more room to breathe and less chance of choking while trying to breathe.
American Kennel Club Standard
According to the American Kennel Club, there are only two different types of Chihuahuas. You might expect that to be apple head and deer head but, actually, there are either long-haired or short-haired Chihuahuas. The breed standard for both states a preference for an apple-shaped head, but the dog is not required to have one to be considered a true Chihuahua. The terms "apple" and "deer head" are basically nicknames that people use to describe the difference.