Cantering Black Horse Picture

Do you know how hard it is to properly expose a black horse with a rider wearing a white helmet under full sun? I think I need to replace either the helmet or the horse.
 

Picture Specs & Equipment Reviews

200mm  f/2.0  1/2500  ISO 100    Size: 3481W x 5222H  
 
Canon EOS 1Ds Mark III 
Canon EF 200mm f/2.0 L IS USM Lens