* Natural Selection: In harsh environments, such as cold climates, mountainous regions, or islands with limited resources, smaller size can be advantageous. Smaller ponies need less food, can maneuver better in difficult terrain, and can better withstand harsh conditions.
* Domestication: Throughout history, humans have selectively bred horses for specific traits, including size. Smaller horses were often favored for tasks like riding children, working in mines, or pulling carts in small spaces.
* Genetics: Specific genetic mutations contribute to smaller size in ponies. These mutations can be influenced by environmental factors, and can be passed down through generations.
* Island Effect: Ponies that evolved on islands often show smaller sizes than their mainland counterparts. This is due to limited resources and the lack of predators, which promotes smaller body sizes.
It's important to note that "pony" is not a distinct species but a size designation. Some breeds are primarily ponies (e.g., Shetland, Welsh), while others can be either horse or pony size (e.g., American Quarter Horse).
Therefore, it's not that ponies "stayed small" but rather that various factors, both natural and man-made, led to the evolution and domestication of small horses that we now call ponies.