What do you call a mature male dog?

The term for a mature male dog is "stud." This designation is typically used in the context of breeding to refer to a male dog that is proven or has the potential to sire offspring. The term is commonly recognized within the canine breeding community and indicates that the dog has met the necessary criteria or characteristics that make it desirable for breeding purposes.

The other terms do not apply in this context. "Tom" is used to refer to a male cat, "rooster" indicates a male chicken, and "buck" is the appropriate term for a male goat. Understanding these specific terms helps clarify the distinctions between different animal species and their respective terminology.