A girl who dresses or acts in a stereotypically boyish way is often called a tomboy. Some famous literary tomboys include Scout from "To Kill a Mockingbird" and Jo from "Little Women."
The word tomboy first appeared some time in the 1500s, meaning "wild, romping girl who acts like a boy." In nineteenth century America, being a tomboy was seen by many as extremely healthy for girls who were otherwise not encouraged to exercise outdoors. Wearing less constrictive clothes and actually playing gave tomboys experiences similar to those of boys — at least before they were expected to grow up and be "feminine."