From A Wiki of Ice and Fire
Maester is a mule owned by Ser Duncan the Tall, a gift from one of Egg's brothers[N 1] at a time when he and Egg owned no other horse than Thunder, having sold Sweetfoot early on and lost Chestnut due to his death in the desert of Dorne on the way to Vaith.
Maester was usually ridden by Egg, until Ser Duncan received Rain, a young palfrey, from Rohanne Webber. The mule is mainly used as a pack animal to carry Ser Duncan's supplies.
- ↑ The mule was almost certainly given by Aemon, as its name is Maester, and no mention is made of Egg encountering his other brothers after the Ashford Tourney, whereas it is stated they met Aemon in Oldtown.