The horse fossil sequence shows how descent with modification might work.
The fossils thought to be the earliest horses are very small and had three toes on the front and four toes on the back.
The next fossils in the sequence are larger and have only three toes on the front and on the back. These ancestors of the modern horse could run faster than than the earlier horses.
The next fossils have three toes on the front and back but two of the toes were short and were not touching the ground when the horse was running allowing the horse to run faster.
The Modern horse is much larger and has only 1 toe both front and back allowing the horse to run very fast.
By losing genetic information and becoming simpler the horse has been able to adapt to the environment and survive better.