Dogs
Rubber traffic cones (or a wooden barrier) placed at certain distances out from the inner rail, when the track is wet, muddy, soft, yielding or heavy, to prevent horses during the workout period from churning the footing along the rail.

Dosage
A system that identifies patterns of ability in horses based on a list of prepotent sires, each of whom is a "superior sire." The Dosage system puts these sires into one of five categories brilliant, intermediate, classic, solid and professional, which quantify speed and stamina.

Dosage Index
A mathematical reduction of the Dosage profile to a number reflecting a horse's potential for speed or stamina. The higher the number, the more likely the horse is suited to be a sprinter. The average Dosage index of all horses is about 4.0.  

Driving
A horse that is all out to win and under strong urging from its jockey.  

Drop Down
A horse meeting a lower class of rival than it had been running against.  

Dwelt
Extremely late in breaking from the gate.