Boreas is one of the Wind divine horses.
You may ride him over oceans to increase his wind strength or over mountains to increase his skills.
Rides over plains increase his skills and his wind strength.
Up until 3100 years old, as soon as the wind strength of Boreas reaches 100 %, it'll be able to generate a storm.
Once the storm is over, you'll either win:
The wind direction determines skills or wind strength points that Boreas is winning during rides.
When the wind is from a certain direction, Boreas might also receive a skills bonus or penalty for the current day.