Tragelaphus eurycerus isaaci. A tricky shot against the light. I am reliably informed that this calf's mother was pregnant when she arrived at Chester.
@Azubaa They have been dwindling, but they are now building up the herd again. I'm not sure how many they have at the moment or how many they aim to have eventually. They are not included in the plans for Grasslands, but they could go elsewhere in they zoo.