Animals can tell each other things without saying a word.

When Fleetfoot gave his head a little toss in a certain way, I knew that he wanted to have a race.
He had a beautiful even gait, and went very swiftly.

Mr.Harry kept speaking to him to check him.
"You don't like him to go too fast, do you ?" said Miss Laura.
"No," he returned.

"I think we could make a racer of him if we liked, but father and I don't go in for fast horses.

There is too much said about fast trotters and race horses.