For domestic horses the answer is yes, no and maybe!

While it is true that wild and free living horses survive without rugs, they move more than their domestic counterparts and are able to find their own shelter from bad weather. On the down side, wild horses do not tend to live as long as domestic horses. Older domestic horses may therefore need extra warmth in cold wet weather but healthy younger horses do not usually need to be rugged especially if they have access to shelter (which all horses should have).

However some breeds of horses have a very fine skin and coat (such as Thoroughbreds and Standardbreds). This means that they feel the wet and cold more than tougher, hardier breeds of horse. There are always exceptions though so treat each horse individually. Rugs should not be used as a substitute for shade and shelter.

What about rugs in summer?

Using rugs on horses in summer can be a welfare issue. Most rugs do not keep horses cool. A horse naturally has a sleek coat which reflects the sun and a horse will seek shade when they are hot as a natural response. All large bodied animals, such as horses, take longer to cool down (and warm up). Rugs prevent any cooling breeze from cooling the body. Also horses are one of the few animals that rely on sweating to cool down and rugs impede this process (by preventing air from passing over the body, evaporating the sweat and cooling the body).

What about rugs in winter?

In cold wet weather a good quality and well fitting rug can help the horse to maintain condition, as a cold wet horse will burn a lot of energy keeping warm. The coat of an unrugged horse stands up in cold weather to trap air and warm the horse. If you decide to rug you have to compensate for this mechanism as a rug will stop the hair from being able to do its job. In some circumstances a rugged horse is actually colder than an unrugged horse if it is a badly fitting thin rug that flattens the hair and reduces the movement of the horse without providing any real warmth.

Rugs need to be checked regularly to make sure that the straps have not broken and the rug slipped, which can cause injury to your horse. A rug should also be removed regularly to make sure it is not rubbing, letting in water etc. and to make sure the horse hasn't lost weight or gained too much weight. Rugs prevent horses from exfoliating their skin properly (by rolling and mutual grooming etc.) so a rugged horse must be groomed thoroughly and frequently to get rid of the build up of dead skin and hair.

The source article can be found here: RSCPA

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