Regular - non magic horses
Foals are born with xp from both parents. The xp is calculated at the time of breeding. Add the mare (also known as dam), and stallion (also known as sire) xp and divide by 50 to get the foal xp. You can also use the foal level tool.
Magical Horses foals have the advantage of getting more birth xp. Add the mare and stallion xp and divide by 20. The foal level tool works for regular horses, use the level / xp tool to calculate level after you have added and divided.