Arthur Waterhouse wrote several children’s books, and also wrote under the pseudonym Vera Painter. As far as I can tell, Dark Champion was his only pony book. It is a fairly conventional horse rescue tale, interwoven with the story of a girl’s mysterious origins. Arthur Waterhouse was perhaps not the world’s greatest expert on horses. According to Hannah Fleetwood, who has written a review of the book, 'I would hazard a guess that he did not really know one end of a horse from another: I found [as] with Peter Grey’s Kit Hunter series, [that] although the story/ies are great there are rather large and gaping blunders where the horses are concerned. In this one, the main thing is [the way] the children gallop and jump Jim straight from the stable and cold. Warming up is never given a second glance and like Peter Grey's books it is assumed/taken for granted that all horses need regular good gallops and can be jumped day after day after day without getting fed up/soured.'
Finding the book
Easy to find in all its printings, and not expensive.
Links and sources