Why is dog one of the great mysteries English language?

"Dog" isn't a mystery in the English language. It's a very common and straightforward word. Perhaps you're thinking of something else?

Could you clarify your question? Perhaps you're thinking about:

* The origins of the word "dog": The word "dog" has a long and fascinating history, with its origins traced back to Proto-Germanic and Proto-Indo-European roots.

* The evolution of dog breeds: The incredible diversity of dog breeds is a fascinating area of study, and the question of how and why they evolved is a subject of ongoing research.

* The relationship between humans and dogs: The bond between humans and dogs is unique and profound, and understanding its origins and impact is a complex and intriguing area of study.

Please provide more context or details about what you're curious about.