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