Final answer:
A mature male horse or pony usually kept only for breeding purposes is called a stallion. Stallions are selected for their desirable traits and high fertility.
Step-by-step explanation:
A mature male horse or pony usually kept only for breeding purposes is called a stallion. Stallions are selected for their desirable traits such as well-built physique and established pedigree, which are indicators of a high degree of fertility. These stallions are used to produce offspring with similar desirable traits for various purposes, including improving the breed or participating in equestrian events.