The Last Unicorn Home by Ali Jaafar Chehab

The book features Courageous, a pony prince poisoned at birth unable to grow his horn, forced out of the kingdom by society so someone else could take his place. Since only unicorns could stay on the island, he must find a way to grow his horn and return to the island. On his search for the ingredients to grow a new horn, he meets a young girl named Penelope, makes various other friends and learns many lessons about life.