Feeding watermelon rind to horses is perfectly acceptable, as long as you give them relatively small pieces with no sharp edges. It is important not to cut longer slices of watermelon to give to horses as they may have problems with swallowing it. Instead it is advisable that you cut them up with smaller pieces so as not to harm them at all.
Watermelon is a very healthy and beneficial food for both humans and many animals, including horses. While watermelon may not be an essential part of a horse’s diet, many of them enjoy the taste and it is perfectly acceptable to feed it to them as long as it is cut into easily-digestible slices.