The Big Book of Lesbian Horse Stories · 5:56pm Jul 2nd, 2016
It exists. It was published by Kensington Books in 2002 and promptly won the Diagram Prize for Oddest Title of the Year, since it was still at least 8 years until lesbian horses would become mainstream entertainment.
Unfortunately, as the few online reviews point out, there are no lesbian horses in the book -- only lesbian humans as well as horses of undefined (and apparently irrelevant) orientation.
I do apologise, but I cannot resist:
66.media.tumblr.com/594779e0675c7576f02bc7f221c8d003/tumblr_ml0fp6AcBT1r1wdbeo1_500.png