Sobekkare's Revenge — Angela Dorsey

An ancient treasure is unearthed from an Egyptian cave, and one of the artefacts carries a terrible curse, a fatal illness that attacks horses. When Jumana discovers her family's prized horses are missing, she follows their tracks into the desert, only to find them with a very strange girl who asks her to undertake a very strange task: stop the curse by returning the artefact to the cave before all the world's horses are afflicted.