The tail as a barometer
A horse's tail is rarely still, and the occasional swish is just fly defence and passing commentary. Constant swishing is different: a fast, repetitive, sometimes forceful lashing that carries on alongside whatever the horse is doing. Because the tail answers to irritation — physical, environmental or emotional — the useful question is not how to stop it but what it is pointing at. Start by noting the pattern honestly: only in summer turnout, only when ridden, only at feed time, or all day regardless. Each version carries its own shortlist of causes, and the pattern narrows it faster than any single examination.
First suspect: flies and skin irritation
In the field, insects are the overwhelming favourite. Horseflies and midges bite around the hindquarters, belly and tail head, and a horse being tormented will swish almost without pause, often stamping and bunching up with companions for relief. Check the skin at the tail head and between the hind legs for rubs, scabs, hair loss or thickened, itchy patches — some horses react far more strongly to midge bites than others and end up rubbing the tail itself broken against fences. Fly protection, a rug made for the purpose where appropriate, stabling through the worst hours and keeping the tail and skin clean all reduce the assault; broken skin that is not healing, or rubbing that is destroying the tail, is a conversation to have with your vet rather than a wait-and-see. A sweet-itch-prone horse is often worst at dawn and dusk, when the midges feed hardest, so the hour of the swishing is itself a clue worth writing down.
When it happens under saddle instead
A horse who swishes mainly when ridden is usually commenting on the work: discomfort from the saddle or girth, a back that is tight, leg and rein aids that arrive too strong or too often, or plain frustration when the questions being asked are confusing or relentless. Watch for the precise moments — at the leg aid, in transitions, when the rein contact is taken up — because the timing tells you what he is objecting to. A horse who also pins his ears, grinds his teeth or resists going forward is painting a fuller picture of the same problem, and the fuller the picture, the less likely it is to be 'just his way'.
What to note, and when to call the vet
Keep a simple log for a fortnight: when the swishing happens, the weather and fly level, whether he was tacked up, and anything else that changed that day. Patterns that feel vague in memory tend to be obvious written down. Bring your vet in if the swishing is new and unexplained, if there is hair loss, broken skin, swelling or a wound anywhere around the tail and hindquarters, if it arrives alongside weight loss or changes in his droppings, or if saddle-time swishing comes with resistance and ear-pinning — those combinations point to something that needs examining, not just observing. That fortnight's log is exactly what belongs in his Pet Passport in KinsLe, travelling with his records so your vet can read the pattern you spotted instead of starting from a blank page.