Árvakr is a horse that pulls the sun's chariot with Alsviðr
When it wakes up, Árvakr leaves a few rays of sunshine behind.
When you've collected enough rays of sunshine, you can enable the bonus: your horses will wake up at 6 instead of 8 for the next 2 hours.
This bonus does not work on Christmas Reindeer horses.
If you also have Alsviðr, your bonus will last twice as long.