Which of the following is a characteristic of the gray horse coat?

The characteristic of a gray horse coat that stands out is the dark mane. As gray horses mature, their coat color often changes from a dark hue to a lighter shade, and their manes typically remain darker throughout their lives, contrasting with the lighter coat. This dark mane is a defining feature that can often help in identifying gray horses.

The other features mentioned, such as blue skin, striped backs, or golden spots, do not generally pertain specifically to gray horses. Blue skin is more associated with certain breeds or conditions and isn't a defining trait of the gray coat. Similarly, striped backs or golden spots are not common characteristics found in gray horses, as they pertain to different color patterns or breeds.