A molera is a common thing with chihuahuas.
80 to 90% of the chihuahuas puppies are born with it.

What is a Molera?
It is an opening in the top of a dog’s skull where the parietal and frontal bones have not fused together.

Most puppies are born with an incomplete skull, the opening of which is called a molera.
It’s found at the top-center of the skull, several inches above the nose.
The molera is typically either circular or diamond shaped, with an average size of 1/4 inch to 1 inch in diameter.
Some have smooth edges, while others have rough or jagged edges.
As the pup matures, cartilage pulls the skeletal plates together, closing the molera and calcifying the skull.

You can compare it with our babies, who are born with an incomplete skull (fontanelle).

Normally, these cranial gaps gradually close over time. But in some cases the molera may never close.
Some Chihuahuas live their entire life with this cranial opening, in which case certain precautions should be taken to protect against injury.

Moleras are very common with Chihuahua puppies. It’s so common, in fact, that it’s mentioned in the Chihuahua’s breed standards by major canine associations throughout the world.

What’s really interesting is that moleras were even more common in Chihuahuas during the breed’s early years. Up until the early-to-mid 1970s, Chihuahuas were bred to be a smaller, with a preferred weight of just 2 to 4 pounds. The preference for smaller Chihuahuas resulted in a higher rate of moleras.

Because of their smaller, dome-shaped skull, apple-head Chihuahuas are more likely to develop and retain a molera than their deer-head counterpart. Teacup Chihuahuas are also more likely to have a molera because they too have smaller heads.

Yes indeed, there are different type of chihuahuas.