I know it grows best on human blood but this is not usually available. Horse and sheep blood are often used in blood agar solutions, but what is it about the blood that makes one preferable to the other?
The factors present in blood of different species are different which also makes inter-species blood transfusion not possible. These provide better growth conditions for any bacteria. As such bacteria have evolved parasitically they then to prefer blood of one species over other. I think this should be the reason for Arcanobacterium haemolyticum growing better on sheep blood than horse blood.