His own people never forgot that he had been a donkey. During his reign, and to his face, he was called Rabadash the Peacemaker, but after his death and behind his back he was called Rabadash the Ridiculous, and if you look him up in a good History of Calormen (try the local library) you will find him under that name. And to this day in Calormene schools, if you do anything unusually stupid, you are very likely to be called "a second Rabadash."
The Horse and His Boy
One of the most feckless villains in Lewis' Chronicles of Narnia, Rabadash was the spoiled, cruel son of the Tisroc of Calormen. He courted Queen Susan of Narnia, who visited Calormen's capital of Tashbaan and saw firsthand his poor matrimonial qualifications. Rabadash intended
Furious, Rabadash intended to imprison Susan and her party. They escaped, however, and Rabadash sought the Tisroc's permission to invade Narnia in retaliation. The Tisroc, seeing a win-win proposition (either capture Narnia or get an overly-ambitious Prince out of the way) agreed. Rabadash took two hundred horse and laid siege to Anvard, the capital of Narnian neighbor Archenland. The Narnians had early warning, however, and met Rabadash at Anvard's gates. In the ensuing battle his army was cut out from under him.
As the Archenlanders and Narnians were debating what to do with Rabadash, the Narnian Lord Aslan appeared before them all. Aslan gave Rabadash to repent, but Rabadash instead ranted loudly. As punishment, Aslan transformed Rabadash into a donkey. To break the spell Rabadash would have to be brought into the temple of Tash in Tashbaan (where he would change back to a human) and never stray more than ten miles from Tashbaan lest the change be permanent.
His adventurousness thus curtailed, Rabadash's future reign as Tisroc was among the most peaceful for its neighbors Narnia and Archenland.