Horse & Pony Books:
FEDORA THE DONKEY
ILLUSTRATED BY IRIS GILLESPIE
SUMMARY: Hard to find donkey story.
THE STALLION FROM THE SEA
SUMMARY: A tale of adventure, smuggling and a chestnut stallion, based on a Manx legend.
HAKI THE SHETLAND PONY
ILLUSTRATED BY VICTOR AMBRUS
Reprinted a number of times in paperback, some with the lovely original illustrations.
SUMMARY: Fed up with life on his parent's croft in Scotland, young Adam sells his Shetland pony Haki to the circus on condition that he will be employed as the pony's trainer.
Haki is very easy to find, mainly in its paperback form. The other two, which I don't think were ever reprinted, are very rare and can be quite expensive to buy.