Mints, G/f's horse loves extra strong mints
Please be wary DO NOT always treat the animal every time you see them, as they can become bolshie. Then can come to expect it everytime some one comes up to see them. In the case of a large horse this can cause issues, treats are to be given when a reward is due, not as a given.
As told to me by my horse owning better half.
use to work weekends at some racing stables in bristol while i was at uni, used to tease the top horse there, give all the horses a couple of mints as you walked down the stable and ignore him, you would get such an expressive response of indignation from him.