Yes — Many Horses Prefer the Rind
Horses can eat melon rind, and watermelon rind in particular has a following: plenty of horses work through a bowl of rind wedges with more enthusiasm than they ever showed the sweet red flesh. The rind is firm, juicy and mildly flavoured, safe from skin to the pale inner layer, and eating it is slow work — which, for a treat, is a virtue rather than a flaw.
Lower Sugar Than the Flesh
The rind's quiet advantage is what it leaves out. Most of a watermelon's sugar sits in the flesh, so the rind delivers the crunch and the moisture at a fraction of the sugar cost. That makes rind a genuinely useful treat for the easy keeper, the cresty pony and any horse whose vet has sugar on the watch list — the animals usually exiled from the treat bucket entirely. Useful is not the same as unlimited: rind is still an extra, offered in wedges, not a bucket feed.
Wash the Outside First
Whatever a melon rolled through in the field or the shop ends up on its skin, and the skin is exactly what you are feeding. Rinse the whole melon under the tap or the yard hose before the knife comes out, and scrub a netted cantaloupe rind properly, because that rough netting traps soil in a way smooth watermelon skin does not. Melons that have been cut and left sitting, or rind that has dried and curled in the sun, go to the compost instead.
Chunks They Can Manage
Cut rind into wedges or chunks sized for the horse in front of you — generous pieces for a big cob who chews thoroughly, smaller ones for a pony, a youngster or an old horse with worn teeth who might be tempted to bolt a large piece. Stay for the first offering of anything new: most horses crunch rind methodically, but the first minutes tell you whether this particular horse treats it as food to savour or a challenge to swallow. Honeydew rind can be served exactly the same way once washed, and wedges pushed into a small-holed haynet stretch the treat into twenty minutes of slow work for a stabled horse on box rest who is watching the yard go by without him.
Fresh Only: the Fermentation Rule
Fruit left lying in a summer paddock ferments, and rind is no exception. Dropped pieces bake in the sun, draw wasps, and turn into something no horse should be eating by the following morning — quite apart from the second horse who finds them. Serve rind at the gate or in the stable where you can see what is eaten, and clear what is refused the same day. Rind day is a treat day, not a scattering exercise.
When melon rind is the standing summer treat for a horse, adding it to his treat list in KinsLe's Care Teams means the person covering your holiday feeds the wedges you approved, washed and cut the way you cut them.