Somatic mutations often appear as irregular patches of color in unexpected areas. Their appearance can sometimes resemble chimerism, and without genetic testing it may be difficult to determine which condition is responsible. However, somatic mutations typically produce asymmetrical or uneven patches rather than evenly distributed patterns. These markings arise when a genetic change occurs in a group of cells during development, causing that localized area to express pigment differently from the rest of the coat. As a result, the mutation may appear as a small white spot where pigment cells failed to develop normally, or as a larger patch of a different color. Somatic mutations are usually not heritable, as they affect only a portion of the body’s cells rather than the horse’s entire genetic makeup. They are often present from birth, although they may become more noticeable as the horse ages.

Birdcatcher spots are small, random white spots, usually no larger than an inch across. They pop up spontaneously on the body of horses of practically any solid color and usually appear later in life. Sometimes the markings seem to move around a bit, vanishing and reappearing elsewhere on the body. They may also disappear entirely over time. Mainly because of this, many believe they are not tied to a permanent genetic marker but perhaps influenced by environment, age, or hormonal changes. The name comes from a Thoroughbred horse named Birdcatcher. Interestingly, Birdcatcher was actually a rabicano, but his white ticking was so prominent that the name "Birdcatcher spots" was later linked to larger white spots, completely unrelated to rabicano. Birdcatcher spots are not heritable.