Swift gallop round the PBOTD today: it's Hilda Boden's Pony Boy, first published in 1958.
Lutterworth, 1958, 1st edn
It's the story of Colin, and his Welsh pony. He calls the beautiful black pony Merlyn. Colin doesn't have a handy instructor lurking to teach him how to look after the pony: he gets a book out of the library so he can learn to ride from it, and acquires tack by cleaning leather. Then he meets Lucy. Lucy helps him win at the Summer Show, but Lucy has an ulterior motive. She wants a pony like Merlyn herself.