I found my little book, it's 24 pages (plus an insert) and paperback: Macrame Horse Tack by Carol Peterson
She had a shop in Riverside, WA, Carol's Custom Macrame, back in the 80s. It's got instructions for making breast collars, headstalls, halters, reins and a lead rope but all for full/average size. Looking at it, I remember why I never tried to make any of it.
If anyone here would like the book, let me know, it would fit in a 8" x 10" manila envelope, and I would be happy for someone to get some use out of it.