Horse & Pony Books:
A PONY IN THE FIELD
(SCHOLASTIC PB [USA] 1991)
SUMMARY: Meg is unhappy: her parents can't afford to buy her the pony she longs for and the snooty girls at the stables where she rides make fun of her. Then her dream comes true when she is given a Shetland called Beauty. But life does not suddenly become perfect now she has a pony, in fact her troubles could just be beginning...
First edition was paperback. It was not reprinted. Easy to find in the USA, a lot harder elsewhere.