Horses Born and Died at Same Time but with Different Ages

Problem
$2$ horses were born at the same time, travelled the world, and died at the same time, but did not live to the same age.

How is this possible?

Solution
This puzzle relies on the definition of age as:
 * the number of days seen

which in turn is defined as:
 * the number of times the sun has risen over the horse.

One horse travels east for its whole life.

Hence, travelling continually towards the sunrise, the time elapsed between consecutive sunrises is shorter than $24$ hours.

The other horse travels west for its whole life.

Hence, travelling continually away from the sunrise, the time elapsed between consecutive sunrises is longer than $24$ hours.

Hence, while being born and dying at the same time, the number of elapsed days they have seen will be different.