If you have ever wondered why the Chihuahua comes in so many shapes and sizes you'll have to understand their past. It was only about a hundred years ago that the Chihuahua wasn't really considered one specific breed of dog.
On one side, it is a breed stemming from the wide variety of native dogs that the Native American peoples kept. On the other side, it has descended from a breed of dogs that were worshiped by the ancient cultures of Mexico.
The melting pot doesn't stop there, because when the the first dog breeders of the Chihuahua came along, they mixed them with other dogs. You could even add more human intervention– to some degree– the intermixing of Spanish dogs from Christopher Columbus' day.
With all that said, it may be easier to understand by two types.
With the deer head, you’re looking at history! Characterized by longer ears and snout– and some times longer legs. This is the older, original breed that some may call the true Aztec dog. These Chihuahuas amazingly have the look that traces back a thousand years or more.
I personally have an affinity for this type– their slender beauty and rich history. Deer heads range in size, shapes and colors. Both Annie and Roxy would be considered small deer heads. Even though they seem to a little more durable in your hands– as compared to the apple head– I never underestimate how fragile they are.
The apple head is the modern day show dog– sometimes called a teacup, miniature, or small Chihuahua. Even though this version was also around a thousand years ago, it has been purposely refined further to conform to the dog standards created in the 1920s.
Many of these have been bred with other small dog breeds to perfect their unique look. The domed or rounded skull and petite size make them the cute, pocket-sized version that holds the record for the world's smallest dog.
My personal experience with apple heads is that they are extremely, let me repeat, extremely delicate and very light weight. I know them to be somewhat hyper, especially as puppies, and very challenging to calm down. The good news is that they are exceptionally affectionate and usually will love everyone they come into contact with.