Line breeding:  Mating together the descendants of one superior animal so that the common ancestor's desirable characteristics will be standardized over successive generations of progeny; the practice by which breed or family characteristics are fixed.

Lope:  A Western version of the canter, using the same foot placement.  Well named for its free, swinging, low-action method.

Loss:  In hunting, the pack is "at loss" when it cannot stay on the scent.

Lunge:  Long rein or rope, an aid in dressage.  The verb is used of training on the lunge (in the ring, circling round the trainer).  (I believe it can also be spelled "longe.")