A horse who bites during girthing or rugging is almost always communicating, not misbehaving: in most cases he is sore somewhere the straps press, he expects to be sore because he has been before, or the girth has been hauled up too fast once too often. Treat the bite as information. Check the physical causes first, slow the whole routine down to stages, and only treat it as a habit to retrain once pain has been honestly ruled out.
Check for a physical reason before anything else
Run your hands over the girth groove, behind the elbows, and along the back and withers where rug straps sit, watching for flinching, heat, swelling, rubs or a dipped, tense reaction as your hand approaches. Try a different girth — wider, softer, shaped away from the elbow — and check the rug's fit, because chest straps that pull and surcingles that hang tight both give a horse something real to object to. A horse whose biting is new, or who has added other signs such as resenting grooming, dipping his back or going short under saddle, needs a vet to look for pain before anyone labels it behaviour.
Slow the routine down to stages
Most girthing problems are made by speed. Do the girth up loosely to the first hole, walk the horse a few steps, take it up one more hole, walk again, and finish shortly before mounting rather than cranking it tight in one go in the stable. Handle the rug the same way — let him see and smell it, lay it on gently rather than throwing it over, and fasten the straps in the same order each time. Stand where he cannot easily reach you while you work, up by the shoulder rather than in biting range of the girth groove, without turning the session into a contest. A small reward after the girth is done, given consistently, gives him something in the routine to anticipate other than discomfort.
Retraining a habit that outlived its cause
If the vet finds nothing and the fit is right, a long-standing snap may simply be a habit the horse rehearsed for months. Habits change through repetition, not punishment: hitting back teaches him the girthing moment is a fight, and he will only get quicker and sneakier about it. Instead, keep the staged routine identical every day, stay calm through an air-snap where it is safe to do so, and reward the days he stands quietly, so the quiet version becomes the routine. Most horses soften within a few weeks of the new pattern. Warn anyone else who handles him not to revert to the quick method on a busy morning, because one rushed session re-teaches the old lesson.
Safety while the work is in progress
Until the behaviour fades, treat it as a known risk: tie him where he cannot swing round, keep other people out of reach while straps are being fastened, and never let children do up the girth of a horse who bites. Log what you find as you go — which girth helps, which stage triggers the reaction, whether the vet found anything — and keep those notes where everyone who tacks him up can see them. KinsLe's Care Teams lets you hold that girthing routine and its notes on his profile for the whole yard to follow, with tasks marked complete by whoever did them, so the careful version becomes the standard one.