Pony Books:

GOLDEN MARE
(COWARD McCANN [USA] 1955)
ILLUSTRATED BY PERS CROWELL
Republished in paperback.
SUMMARY: Robin, recovering from an illness, is a lonely boy who finds solace in his closeness with Magic, the golden palomino mare. Although she is too old to do much he loves her deeply. Then one day he and his horse face a dangerous adventure which could change Robin's life forever.

PONY FOR KEEPS 
(COWARD McCANN [USA] 1958)
ILLUSTRATED BY PETER BUCHARD
Rare. I don't think there was a paperback edition published.
SUMMARY: Animal-mad Katty-Lou is lonely at her foster parents until she finds out that there is a horse farm nearby. But her foster parents have forbidden her to visit itů.

HORSE IN THE HOUSE
(COWARD McCANN [USA] 1964)
(1st UK edition METHUEN 1966)
ILLUSTRATED BY SAM SAVITT
Republished by Methuen in 1966 and as a Puffin paperback.
Reprinted in the UK in hardback and paperback.
SUMMARY: Melanie is mad about her palomino Orbit. When her parents go away she thinks it's a great idea to see if he will come into the house with her. But when she tells her schoolfriends about her plans it sets off a chain of events which could put Orbit in danger!
PONYMAD RATING: 5 HORSESHOES

Read/join in with a discussion of the book in the chat forum reading circle

There was also TV drama series based on the book and scripted by Rosemary Anne Sisson.

Collectors Info:
Horse in the House (the paperback at least) is by far the easiest one to find and is the only one commonly seen in the UK. Hardbacks can be harder to track down. Golden Mare is also reasonably easy to get hold of on internet book sites. Pony For Keeps however is a lot rarer and more expensive to buy, especially outside of America.