What is the term for a mature male horse?

A mature male horse is referred to as a stallion. The term "stallion" specifically denotes a male horse that has not been castrated and is capable of breeding. Stallions are often recognized for their strong physical characteristics and are used for breeding purposes to produce foals.

The other terms do not apply in this context. A "stud" typically refers to a stallion used specifically for breeding, and while it can be related, it is more specific than just identifying the animal itself as a stallion. "Rooster" and "chick" are terms used to describe male and young chickens, respectively, and are entirely unrelated to equine terminology. Being familiar with these terms helps in understanding the terminology used in animal science and breeding discussions.