Mary Adrian (1908 - ) was an American author. Her book The American Mustang was part of a series on American animals, aimed at the preservation of threatened species. Other titles included American Alligator and American Prairie Chicken -this last not quite such a strange thing as it might sound to British ears. She also wrote several other animal-based books for children, as well as a mystery series, of which her other horse book, The Indian Horse Mystery, is part.
Finding the books: both books are easy to find in the USA. Neither was published in the UK.
Links and sources
Terri A Wear: Horse Stories, an Annotated Bibliography, Scarecrow Press, 1987
Many thanks to Lisa Catz for the photograph.
The Indian Horse Mystery
Hastings House, New York, 1966, 126 pp, illus Lloyd Coe
Hap, Kathie and Yakima Indian Jim are investigating the disappearance of cattle from the Hanging Rock Ranch. This is all mixed up with Jim trying to tame a wild grey mare.
Bibliography: horse books only
The American Mustang
Hastings House, New York; 1964., 61 pp, illus Genevieve Vaughan-Jackson
This I think is non fiction, and is the story of a mustang from foalhood to maturity.