Ihuarraquax is a young pure white unicorn who wandered the Korath desert with Ciri for a few days. She saved him after he was poisoned by a sand monster by using the Force found in the element of fire, something Yennefer had expressly forbidden her to do. When they met, Ihuarraquax was really only a colt and Ciri named him "Horsey".
Between them, they were able to find food and water for each other and likely neither would have survived the desert without the other.
In Pani Jeziora
Ciri was reunited with a now fully grown Ihuarraquax, though she continued to call him Horsey. He helped her escape the clutches of Eredin and the Dearg Ruadhri.