In West Virginia the term fairy diddle refers to a near-mythic creature, and may be used for various woodland rodents, including tree squirrels, flying squirrels, ground squirrels, and even baby groundhogs. Most observers agree, however, that whatever the fairy diddle is, it is fast, smaller than the average tree squirrel, and makes a lot of noise if disturbed. This accurately describes the American red squirrel, also called the pine squirrel or chickaree. Thus in West Virginia fairy diddles are most often red squirrels. Half the size of the gray squirrel, the red squirrel is omnivorous and will attack and eat other small mammals. This behavior may have given rise to the myth, common in West Virginia, that fairy diddles raid the nests of other squirrels and castrate their young, creating so-called steer squirrels. Other West Virginians identify the fairy diddle itself as such a steer squirrel. Naturalists disallow both of these assertions.

Fairy diddle as a term is most often used in northern West Virginia. It may be spelled alternately fairydiddle, ferrydiddle, and ferrididdle. In the southern region, "mountain boomer" is used synonymously. Because of its association with rapid movement the term is often used descriptively, as in "quick as a fairy diddle."