It's not the usual tale of girl gets pony. Girl does get the pony, but it's totally unsuitable, and is in fact a horse - a thoroughbred, the sort of horse of which a horse mad girl dreams that she, only she, will be the one to find the key to poor, spoiled, dangerous him. Unfortunately for heroine Lucy, she doesn't have the backing and advice of a practical, sensible, and yes, moral, Mrs Darcy. Lucy has Janey Squires, who owns the riding school at which Lucy's going to keep her horse.
Janey has a vicious rivalry (from her side at least) with rival riding school owner Angelica Kent. Janey knows full well Lucy's horse Dance is unsuitable, but she encourages Lucy in the hope that she'll show Angelica what's what by winning a competition, and, for once, trouncing Angelica.
Janey is one of those who likes to draw all those around her into her wars, and she does that with the girls at the riding school. The book's really about that: the dangers of obsession, and the inability to see the truth when it's under your nose. Lucy, in the end, starts to realise that just because an adult says something, it isn't necessarily so, and she starts to make up her own mind. She's helped here by the fact that Angelica's a much better horsewoman than Janey, and it's that, in the end, that changes her mind. She chooses the welfare of her horse.
Sadly, there were no other horse books from Jill Maughan, but the one she did write is well worth seeking out. The Deceivers was published by Armada in 1990, and was an Armada original. As far as I'm aware, it had no further editions.
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For more on Jill Maughan (there's not much, but you might enjoy the page), see my website.