I came to cycling as an equestrienne, so I'll disclose immediately that my tastes may be biased.
Bike commuters and equestrians both require a helmet with a classic appearance, good ventilation, and comfort. As long as the horse helmets are ASTM/SEI Certified I don't see any reason why a person couldn't wear one to commute by bike. They're warmer than traditional bike helmets, which could be a problem for someone in Florida but isn't usually a problem in Boston. Horse helmets have more protection to the back of the head. No big deal there.
My position on this is that the best helmet is the helmet that you're willing to actually wear. Period. For any sport.
There are numerous equestrian helmets available from Dover Saddlery and State Line Tack.
Rising helmets have had longer to evolve so they're also available in purple velvet, tan leather, and other fun options.
