Yes — a Cool, Mild Crunch
Horses can eat cucumber, and on a hot afternoon a cold spear of it, mostly water and crunch, is a refreshing change from the usual bucket. Nothing in a cucumber — skin, flesh or seeds — is a problem for a horse. It is a gentle food in every direction: gentle flavour, gentle sugar, gentle on the gut in treat-sized amounts. The care it needs from you is the same care most treats need, and it starts with the knife.
Spears, Not Rounds
Cut across a cucumber and you produce the same trouble as a carrot coin: smooth round discs, exactly the shape that can lodge in the gullet of a horse who grabs and swallows. Choke is uncommon but serious, and it is almost entirely a problem of shape. Lengthwise spears — a cucumber halved, then quartered along its length — break up properly under the teeth and travel down as they should. For a very keen youngster or an old horse with worn teeth, spears also slow the eating down to a civilised pace.
Low Sugar, High Water: Where Cucumber Earns Its Place
Cucumber's real credential is what is barely in it. The sugar content is minimal next to apples, carrots or bananas, so the weight-watching horse, the cresty pony and the veteran on a careful diet can all have a spear without it counting against the day's budget. The water content pulls the other way in summer: a horse who has worked and is drinking well anyway still enjoys the cool moisture of a chilled spear, and it is a friendly way to offer something fresh to a horse on box rest who is watching the yard go by.
The Horses Who Turn Their Noses Up
Be prepared for a shrug. Cucumber's flavour is so mild that some horses take one experimental bite, let it fall, and look at you for the real treat. That is a perfectly acceptable answer — there is nothing a cucumber provides that his hay and feed do not. Offer a spear alongside a familiar handful of chopped carrot the first time if you want to give it a fair trial, and if he still declines, retire cucumber from his list without a second thought.
Serving Notes
Wash the skin before slicing, keep portions to a few spears at a time, and fish uneaten pieces out of the bedding rather than leaving them to go slimy in the stable. Introduce cucumber on a normal day, not on the day three other new things change, so if his droppings loosen you know the likely culprit. Home-grown cucumbers that have grown bitter are best composted — the horse will almost certainly agree with that judgement. Spears kept in a cool box are a welcome sight in a trailer or at a show on a hot day, when a familiar, juicy mouthful settles a horse who is off his water away from home.
Adding cucumber spears to a horse's listed treats in KinsLe's Care Teams means the helper covering a hot weekend offers the same cool, low-sugar extra you would have chosen yourself.