What does mountain goat symbolize in the glass castle?

A mountain goat is Rex's nickname for his favorite child. The nickname refers to Jeannette's endurance in face of trouble. Like a mountain goat, she is able to climb mountains without losing her footing. Jeannette is the only child given a nickname by her father. The endearment implies a special relationship between her and her father that the other children could not share with Rex. Additionally, the nickname foreshadows Jeannette's persistence and endurance when she realizes that her and her siblings must live apart from their parents if they are to ever lead stable, fulfilling lives.