The idea that Chihuahuas have holes in their skulls is a common misconception, likely stemming from the fact that they have a *soft spot* on their heads. This soft spot is called the fontanelle, and it's actually a normal part of puppy development in many dog breeds, including Chihuahuas.
The fontanelle is a membrane-covered space in the skull that allows the skull to grow and expand. It usually closes up as the puppy grows older.
So, while Chihuahuas have a soft spot on their heads, it's not a hole in their skull, and it's not a unique feature to their breed.