The mucus plug in dogs is a thick, viscous, and gelatinous substance that is typically clear or white in color. It varies in size and shape depending on the stage of pregnancy, but generally becomes larger and more noticeable as labor approaches. The mucus plug serves as an important indicator of impending labor, as its expulsion often occurs within the final days or hours before the onset of contractions. It is essential to consult a veterinarian if you observe any abnormal discharge, as this may indicate an underlying medical condition.