horse responding to certain rider

Question
hello sir, lets suppose a mean racehorse got used to one particular jockey. he willingly enters into the starting gate stall with that rider atop. lets  suppose that rider got declared to ride that horse in one particular race but was medically unfit to ride that horse. the trainer substituted another rider. now, will the horse enter into the gates willingly or troubles a lot as new rider sitting astride?

Answer
There is really no way to predict how a horse will react to a new rider. He may be suspected to act out, but not necessarily. The horse may tend to like certain personalities and certain types of holds. The horse may like a very quiet rider who takes a light hold or he may need an aggressive rider who keeps his attention. It's really hard to say unless you know the horse well.