Starting in chapter 18 of the novel and reading the excerpt you cited, it would be difficult to discern who John Manly is. This character is first mentioned in chapter four, when Black Beauty is sold to Squire Gordon when he is four years old. In the chapter entitled "Birtwick Park," Black Beauty meets Merrylegs and Ginger. Merrylegs, a wise old pony, is telling Black Beauty about his ill-tempered new roommate (Ginger) and the caregivers at Birtwick Park, John Manly and James. When explaining Ginger's bad habit of biting, Merrylegs says this about John:
"She [Ginger] says no one was ever kind to her, and why should she not bite? Of course, it is a very bad habit; but I am sure, if all she says be true, she must have been very ill-used before she came here. John does all he can to please her, and James does all he can, and our master never uses a whip if a horse acts right; so I think she might be good-tempered here. You see," he said, with a wise look, "I am twelve years old; I know a great deal, and I can tell you there is not a better place for a horse all round the country than this. John is the best groom that ever was; he has been here fourteen years; and you never saw such a kind boy as James is; so that it is all Ginger's own fault that she did not stay in that box."
Grooms were responsible for all the care concerning the horses—mucking out stables, feeding and watering the horses, grooming them, and preparing them for riding or driving. As stated in the above quote, John tries hard to care for all the horses' needs and is kind. He is an experienced groom, having been at Birtwick Park for fourteen years.
Readers learn more about John in chapter seventeen, entitled "John Manly's Talk." In the excerpt cited in your question, John is riding Black Beauty to summon a doctor because Mrs. Gordon is very ill. Notice in the excerpt you've read that John, even under urgent circumstances, encourages Black Beauty and doesn't ask too much of the horse. Black Beauty states that he went even faster than John asked, of his own accord. This shows the kindness and gentleness of John, which is a theme throughout the novel. There are kind and gentle men and women juxtaposed against cruel and ignorant people.