SARA HERBERT
British author of three mystery/adventure type pony books.

Pony Books:

THE GREAT PONY RAID
(MAX PARRISH 1967)
SUMMARY: Joan and Tom are on holiday at Pony Camp. They make friends with a mare and foal but when the horses go missing they begin to suspect foul play. The pair then round up all the children from the local pony camps in order to foil a gang of ruthless horse rustlers from stealing all the wild moorland ponies.

THE PONY PLOT
(MAX PARRISH 1967)
ILLUSTRATED BY MICHAEL WHITTLESEA
Reprinted in paperback by Tandem in the 1970s.
SUMMARY: Adventure pony story

THE SECRET OF THE MISSING FOAL
(TANDEM PB 1975)
SUMMARY: A group of children arrive at their recently deceased Uncle's house to stay and soon become involved in a mystery. Can they follow the clues their Uncle left him and solve the mystery of his death and the missing foal?

Collectors Info:
Not as easy to find as many pony books, the two which were published in paperback are  a lot easier to come across than The Great Pony Raid which as far as I am aware was not republished at all and is only available as the hardback first edition.