I touched on this in a post long ago, but it's worth a more detailed exploration. Thoroughbreds - yes, the speedy horses of racing fame - don't just come in solid colors like bay, gray, black and chestnut, although those are the more common colors.
Sabino and overo are also found, as are occasional oddball colors such as brindle and strange spots. The latter tend to be one-time individual mutations, but sabino and overo can be passed on, and come in a wide array of variations, from a few white markings to wild speckly, roany patterns.
Visit these sites to see some great photos of fantastically colored Thoroughbreds:
Cool and Unusual Thoroughbreds - many pages of photos of wild Thoroughbred colors
Overo Thoroughbreds - another roundup of photos of many colorful Thoroughbreds
Shadow Mountain Stables - breeders of sabino Thoroughbreds
Color World Ranch - breeders of overo Thoroughbreds
White Horse Productions - a good collection of photos and descriptions of colorful Thoroughbreds
Gestüt Falkenhorst - breeders with an overo Thoroughbred stallion
Painted Desert Farm - breeders of sabino and overo Thoroughbreds
Blazing Colours Farm - breeders of sabino and palomino Thoroughbreds