I think it's really easy to understand.
because its like the sun circle - or day circle...
The first epps is the daybreak,
the first pps is the morning,
the full grown horse is the midday,
the second pps is the evening,
and the secons pps is the night.
because to start the next day you have to start with a epps. xD
so it grows backwards from the "midday" horse.
Like the sun goes down every day.
