From the Bicycle Safety Institute website:

http://www.bhsi.org/other.htm

Equestrian Helmets

The equestrian community developed an ASTM standard in 1988 and it has unique characteristics related to horseback riding and horse sports. In particular, the hazard of being kicked by the horse is unique. If you ride in woods, the larger vents of the bicycle helmet are more likely to snag a branch, another hazard that increases because the rider position is higher on a horse than on a bicycle. As a result, ASTM has a specific equestrian standard, and the design of the helmet is for the impacts encountered in horseback riding. Although some riders do wear bicycle helmets for cost reasons, they can have much better protection with an equestrian helmet designed for their sport. It should meet the ASTM standard for equestrian helmets (F1163). There is no law in the US that prohibits a merchant from selling equestrian helmets that do not meet any standard, so look for an ASTM F-1163 sticker. We do not recommend equestrian helmets for bicycle riding because the lab drop test on flat surfaces is done at only 1.8 meters rather than the 2.0 meter bicycle requirement. In addition, most equestrian helmets are not as well ventilated as most bike helmets for summer riding. Plantation, Florida, has a helmet ordinance requiring ASTM F1163 or its equestrian helmet equivalent as approved by the Chief of Police for age 16 and under. It took effect in 1999. New York State now has an equestrian helmet law as well for riders under 14. Its effective date was 1/5/00. It requires an ASTM helmet and also requires that a helmet be provided if you rent a horse from a horse rental service. For sources of information on equestrian helmets, see this message by Dru Malavase, an equestrian helmet expert.

Bicycle Helmets

Bicycle helmets show up now in an amazing number of activities, partly because there are more of them in consumers' hands and more in daily use than any other helmet. You may be asking if a bike helmet is adequate or necessary for bungi jumping, skydiving, snowmobiling, mountaineering, spelunking, jousting, construction work, car surfing, baseball, football, hockey, tricycles or extreme tiddlywinks. The answer in most of those cases is that if there is a helmet designed specifically for the activity it usually will offer more protection for that activity, and is optimized for the type of hazard encountered in the activity, rather than the type of hazards encountered in bicycling. (Tricycling and roller skating are essentially the same activity as bicycling, so bicycle helmets fit there.) Bicycle helmets are narrowly focused on providing protection for one single hard impact on a very hard, completely unyielding surface, while still minimizing weight and providing ventilation. The design tradeoffs that produce a good bicycle helmet may not produce the best helmet for your sport. It may seem ok to you to accept inadequate protection to try a new activity without buying a new helmet, but we would not recommend that. We recommend that you try renting the helmet you really need, or borrowing one from a friend. The "savings" from using a bike helmet could bring you a lifetime of sorrow and a brain that never works as well again. But if you have no practical alternatives, a bicycle helmet is likely to be better than a bare head. Even the family we heard from who had used theirs while sitting in their basement during a tornado warning might have gotten some help if objects had started blowing around!