James Aldridge (Harold Edward James Aldridge, b. 1918) was born in Australia, though now lives in London.  He was a journalist and war correspondent before he started writing full time.  His Quayle family sequence is set in a small town on the Murray River, and his books tend to reflect the Australia in which he grew up.  Stephanie Nettell said:

“Horses are a recurrent Aldridge motif.  The Broken Saddle is very much the story of his own relationship with one special pony as a boy, with the same fierce feeling for the wild country of Australia as in The True Story of Spit McPhee.”

According to Austlit, he is “widely admired for his descriptions of rural Australia and his explorations of the social tensions experienced in country towns.”

Probably his best known book is Ride a Wild Pony (originally published as A Sporting Proposition).  It was made into a film in 1975 with the title Ride a Wild Pony, and Penguin published it under this title in paperback.  

Sources:

Contemporary Children’s Authors, ed. Chevalier, 1989 3rd edn.

The Broken Saddle
Julia MacRae, 1982
Puffin pb 1984

A Sporting Proposition/Ride a Wild Pony
Hamish Hamilton, 1973

Puffin, Australia, 1975, pb, 173 pp.
Republished as Ride a Wild Pony, Penguin pb 1976

The Marvellous Mongolian
Macmillan 1974
Pan Books, 1976

To Baryut, Tachi is a marvellous stallion, roaming Mongolia.  Kittys favourite is Peep, her Sheltla.d  Tachi is
imported to the Welsh nature reserve on which Kitty lives, and Peep is to be his companion.  Kitty is terribly
worried in case Peep is hurt, and Baryut worries that Tachi cannnot be contained in the Welsh hills.
 

Bibliography - Pony Books Only