He'll join your breeding farm once you've sent a horse aged 25 or more to heaven.
Yggdrasil is a special horse that ages every time you send a horse aged 25 or more to heaven.
As he gets older, his appearance changes! After the very first change of appearance, a photo album becomes available so you can look at the horse's previous looks.
Every time he changes his appearance, a gift appears in your profile page.
When he reaches his last transformation, you can rejuvenate him to the age of a few hours.