Katharine Newlin Burt (1882-1977)

American author. She and her husband (fellow writer Maxwell Struthers Burt) ran their own dude ranch in Wyoming and a few of Ms. Burt's novels were set in the ranching world, although most were romances. She wrote for both adults and children/young adults. She appears to have written only the one horse story.

Katharine Burt's son, Nathaniel Burt, was also a writer.

Horse & Pony Books:

SMARTY
(FUNK & WAGNALLS [USA] 1965)
ILLUSTRATED BY VIC DONAHUE
Reprinted in paperback by Young Readers Press in  1968 and 1970.
SUMMARY: Jenny and Stormy squabble over the ownership of the buckskin colt Smarty. Can they come to an agreement on what is best for the horse?

Collector's Info:
Fairly easy to find in the USA and usually quite cheap, but can be hard to get hold of elsewhere.