ChunTian:
Oh...I was under the impression that Xixi can ride his own horse. In TTEOTM, he rode into town to depose his brother and take over the throne. EP 10 Around 17 min 30s mark. There was no assistant next to him leading in the horse. Usually the actors who cannot ride will have someone lead the horse in as a guard by their side or something.
Or maybe for some other more complicated scenes he needs a dummy horse.
There is one scene which needed the horse to gallop so I think that was why he need a dummy horse for closeup. The scene where he rode the horse to the ship or something. I played many times to confirm. I could be wrong. You are right that scene he rode the horse himself leading the guards.