An "undercoat" is a layer of paint, fur, or material applied beneath the top layer. It helps to provide a base and additional protection for the surface or to keep warm in the case of animals. Full definition
The coat has a thick undercoat of soft, dense wool and a longer outer coat of harsh hair that stands out from the body, creating a fluffy look. (dogbreedstandards.com)
With small, pointed, erect ears over bright, intelligent eyes, the Aussie has a harsh, straight outer coat and a short, soft undercoat with a thick ruff of hair around the neck and slightly feathered forelegs. (soonervet.net)
The thick undercoat of this canine made it able to withstand the frigid temperatures when it was out protecting and driving a flock of sheep. (blog.pawedin.com)