In 211 AC, after Ser Duncan the Tall did a service for Lady Rohanne Webber, she intended to pay him back with a gift of a horse. Although Dunk rejected her gift of Flame, he discovered that his squire, Egg, was riding a new handsome palfrey. Egg named the horse Rain, likely for the rains that had just ended the two-year summer drought.
In 212 AC, when Duncan lost a tilt at the Whitewalls tourney to Ser Uthor Underleaf, and could not afford to pay the ransom for his horse and armor, Egg suggested that Dunk could ride Rain, and he would go back to riding Maester the mule. However, Dunk rejected the idea, as that would mean returning in shame to Egg (actually Prince Aegon Targaryen)'s father Prince Maekar at Summerhall.
After Dunk discovered that the tourney was secretly a rendezvous of those fomenting the second Blackfyre Rebellion, he told Egg to go with Lord Ambrose Butterwell, but to give Rain his head and get away to Maidenpool before Butterwell switched sides again.