How did the modern day horse evolve?

Modern horses evolved from perissodactyl ancestors (hoofed mammals with an odd number of toes). One of the earliest ancestors of the modern horse was hyracotherium. It was more or less the size of a terrier and had odd toes but no real hooves yet. Which each successive species the middle toe got larger and larger while the four other toes got smaller, and that is how modern horses have one big hoof on each foot.