Poverty lines are a group of muscles on either side of a horse's rump near the root of the tail. They are between two muscle groups called the biceps femoris and the semitendonitis.
Appearance of poverty line grooves on the rump often indicates that the horse is underweight. If the vertebra can be seen and felt along the horse's rump, then the horse is defiantly too thin.
Poverty lines normally cannot be seen in a horse's rump. But when they are seen, they cause a dark groove that starts near the base of the tail and extends down the foreleg and over the hock, making the leg bones appear more prominently.
Some breeds of horses are naturally thin and may show dark poverty lines even when they are physically fit, according to "The Horse in Motion." These breeds include the thoroughbred and the Akhal-Teke.