Mavis Thorpe Clark (1909-1999) was born in Melbourne in 1909, and died there in 1999.  Her first published work was a serial in the Australasian when she was 14, and she published Hatherley’s First Fifteen at the age of 18.  She won the Book of the Year Award of the Children's Book Council of Australia for The Min Min in 1966.  Her The Pony from Tarella was published by Knight books in paperback in 1977.  As far as I know, this is her only pony book.

Sources:
Austlit

Mavis Thorpe Clark

Pony from Tarella
Heinemann, Melbourne, Victoria, 1959
Paperback:  Knight, Leicester, 1977

Right: 1st Australian ed.
Heinemann 1959
cover art and illustrations by Jean W. Rowe
*price on the dj flap is listed as "price in Australia" and
the copyright pages note that the book is "Wholly set up,
printed and bound in Australia by Wilke & Co. Ltd."

Bibliography - Pony Books Only