If any live in an area that grows P-nuts (I do) you can rest assured that they LOVE those.
There is a sheller facility near me and yrs back we used to drive a dump truck over and they let us put it under the chute that blew out the shells. When full (1 day or 2) we'd go get them and dump into the field in a pile. Had to close them out of the field while we dumped or they would get in the way trying to eat and be covered. But the shells are actually quite good fiber with a good amount of protein. Your horses will eat them also.
Talk about "free feed", what a deal! The P-nut sheller loved us, as they had to spead out this enormous pile and we helped a little with that. Had 100+ goats at the time, so everyone was happy with the situation. Collecting them at once gave us fresh product. With horses, as with all their feeds, you must be vigilant with mold issues when feeding in bulk.