Back at the knee; over the knee:  Position of the knee, indicating weakness of the tendons.  The knee inclines forwards or backwards from the vertical.

Barren:  A mare, neither pregnant nor with foal at foot.

Bay:  A color term for a brown horse whose points (bottom part of legs, mane, and tail) are black.  Bays may range from a medium brown to almost black (called a seal bay).  A red horse with black points is called a blood bay.

Blank, to draw blank:  In hunting, to fail to find a fox.

Blood horse:  A term sometimes used for a horse of the Thoroughbred breed.

Brachymorphic:  A group of classification in which may be found breeds that are traditionally called cold-blooded, especially heavy workhorses like the Clydesdale.

Bridle:  The leather straps that fit onto the horse's head to keep the bit in place.  The bit is the metal part that goes through the horse's mouth.  The reins are the connection to the rider.

Brisket:  The breast or lower part of the chest (of an equine) in front of, between, and slightly behind the forelegs.  From the Middle English brusket, akin to breast.

Broodmare:  A female horse whose job is to have foals.

Buckskin:  A color term for a light brown horse whose points are black.  The color of the body may range from a deep gold to sandy.  Buckskins may also have a dorsal stripe (a stripe that runs from wither to tail).