Answer:
Evolution by natural selection
Step-by-step explanation:
Most probably, the trait of being small, weak and slower did not convey advantage (or was even a disadvantage) in survival in their environment. The bigger, stronger and faster horses had an advantage and were able to survive long enough to pass down their genes to the next generation, Over succeeding generations more and more horses, therefore, became strong, bigger and faster. This is the process of natural selection at work.