As with household pets, like cats and dogs, humans play a large role in the mating and breeding processes of horses. Female horses are typically their most fertile for mating in the summer months. However, it is not necessarily unusual for horses to be bred at any time in the year.
"Horses" by Atli Harðarson;
used with permission under CC BY 2.0
It is a mare and stallion that mate to create a foal. Horses younger than four, like fillies and colts, do not mate due to their body size. And geldings cannot mate.
Horse breeds are often referred to either as “hot-blooded,” “cold-blooded,” or “warm-blooded.” Hot blood breeds, like the now-extinct Turkoman horse, are bred for their strength, speed, and endurance. Cold bloods, like the well-known and very popular Clydesdale, are desired for their patient, calm, and friendly natures. (Although other cold blood breeds are livelier and some are very strong, the distinction between hot and cold is helpful.)
Think…
Hot-blooded ---> HOT ---> fiery, fast, and energetic, like molten hot lava
Cold-blooded ---> COLD ---> more tranquil and sturdy, like a big beautiful frozen lake
Now, warm-blooded horse breeds are a cross between cold bloods and hot bloods. These horses are bred for various combinations of temperament, physical characteristics, and attributes.