The first bit a horse carries in its mouth when beginning training should be as mild and comfortable as possible. The first bit may not be the last type of bit will ever carry. Different types of bits have different purposes. Down the road, you may want your horse to carry something beyond a simple snaffle. If you’re headed for the dressage ring, your horse will eventually carry both a snaffle and curb bit at the same time.
If you show western, your horse may wear any number of western curb bits.
Many people do not want their horse to carry a bit at all. That’s a personal choice. But, it’s wise to consider that in spite of our best intentions to keep our horses forever, dire circumstances may mean you may have to part with them, and they’ll appeal to a wider number of people, and have a better chance at good homes if they are able to go in both a bitted and bitless bridle. So, some bit training is a good idea, even if you personally do not want to use one.
When you are choosing a bit to start a young horse there are a few things to consider. A young horse will chew and champ on the bit, and perhaps at first rub its head to get rid of this new and annoying thing in its mouth. You'll want to choose a bit that is both as safe as possible and comfortable. Logically then a simple snaffle is the best choice. Leave any type of curb to more advanced training.
The first choice will probably be a jointed snaffle bit with smallish rings that would be unlikely to catch on anything if the horse does try to rub its face. You'll also want to avoid spoon or full-cheeked snaffles for the same reason. An eggbutt snaffle with small rings would be a good choice although you might also choose a loose ring snaffle.
Some people use mylar or similar synthetic mouth pieces (some are even flavored). But quite often these mouth pieces are thicker than a metal bit, and can be quite bulky in a young horse's mouth. You will probably find lots of these bits with chew marks on them and metal bit may be less likely to encourage a horse to actually chew on the bit. If you want to add flavor to make biting a more pleasant experience its easy to smear on a bit of molasses, jam or honey. If you do choose a synthetic bit and a horse chews it, get rid of the bit before there is a chance the metal core is exposed.
Because small rings can be pulled through the horse's mouth, the first bit your horse wears may not be the first bit it is ridden in. But choosing a bit with a similar mouth piece makes the transition easier. A curb bit is not a good choice for teaching a horse to carry a bit. Because the curb bit acts on the horse’s mouth, chin and head, it can be overwhelming. A well-fitted snaffle bit will only act on the horse’s mouth. The shanks are also a hazard if the horse tries to rub the bit, or its head. They could easily get caught and the horse’s first experience could be a painful and frightening one.
If you're starting your young horse in the winter time, make sure the bit is warm.
A cold frosty bit is not a nice experience, even for a well-trained horse. Make sure the bit is properly adjusted and fits the horse well. Too low or too high will cause discomfort and you want to encourage your horse to hold the bit quietly.