A greyhound is a dog with a thin body and long thin legs, which can run very fast. Greyhounds sometimes run in races and people bet on them. a type of thin dog that can run very fast and is used in races. or grayhound Fastest dog, one of the oldest breeds (dating from about 3000 BC in Egypt), and long symbolic of the aristocracy. It has a narrow head; long neck; deep chest; long, muscular hindquarters; a long, slim tail; and a short, smooth coat, of various colors. It stands 25-27 in. (64-69 cm) high and weighs 60-70 lbs (27-32 kg). Streamlined and slender, it can reach a speed of about 45 mph (72 kph). Greyhounds hunt by sight and may be used to hunt hares, deer, foxes, and small game. They are frequently raced for sport