The ant is a six-legged insect that crawls quickly on the ground.

six-legged 是由数词 six（六）和名词 leg（腿）加上后缀 -ed 组成的形容词。leg 指"腿"，-ed 后缀表示"具有...特征的"，所以 six-legged 意思是"有六条腿的"。