Most horses living in cold climates resemble wooly mammoths at this point in the season. Even if your horse is blanketed, he still may be quite furry. If he's in steady work and will be for the rest of the winter, you may want to clip him, as long as you plan to keep a blanket on him.
Most horses clip just fine. But if your horse has not been exposed to clippers before, he may take exception to cold steel teeth raking his nice, warm coat. Don't be surprised. There are ways to convince him it's not really a giant mutant, buzzing fly.
Horses object to clippers for different reasons. With some, it's the noise; with others, the sight of their hair falling in great tufts confuses them. If the blades are dull, the clipping experience may be painful, making him reluctant to repeat the process. Or he may have received a shock.
It's always better to avoid using restraints such as twitches. Although a restraint may allow you to get the job done once, the experience will solidify the horse's impression that repeating the process is to be avoided. It's much better to eliminate his anxiety.
It's important to determine exactly what it is about the process that frightens him so you can deal with it. Here are a few suggestions:
If it's the clippers he's fretting about, try to resolve the problem gradually – well before you actually want to clip him. Introduce the feeling of clippers on his body by running them over his coat without turning them on for a few days. Be matter of fact, as you would if you were brushing him. If he objects to having this done, cover the clippers with a cloth and just rub it over his body.
Next, hold the clippers in one hand and the cloth in the other until you can stroke him with the clippers alone. Repeat this process for as many days as it takes for him to become comfortable with it.
Before grooming him, have someone turn on the clippers at some distance from him. After brushing him a bit, cover the clippers again with a cloth and, while they're running, approach him gently. Touch just his shoulder with your hand – not the vibrating clippers. If he appears frightened, stop, back up and wait a few minutes, until you can approach him again. Once you're successful, call it a day.
After a few days of this regimen – once he's comfortable – "groom" him with the cloth-covered clippers first and then gently touch him with the clippers uncovered. Don't touch him yet with the blades, just with the clippers. Run the clippers once again over his body, without getting close to his face. Make sure your clippers aren't hot.
Some horses will allow you to clip their bodies, but hate having their bridle paths or ears done. This can be accomplished with scissors until you are able to establish a comfort level with the clippers. It's most important that he allow you to body clip him. Work your way up from the horse's leg to his shoulders to his poll. Don't attempt to clip the ears or face of a skittish horse until you have firmly established a clipping routine on his body. It only takes one negative experience for your horse to decide this isn't such a good idea, after all.
The key here, like most unfamiliar activities for equines, is patience and making the procedure as pleasant as possible. If you are gentle and persistent, most horses are quite willing to learn.