A fieldmouse is a small mouse with a long tail that lives in fields, hedges, and the edges of woods — not in your kitchen. In British English it is often written as two words, field mouse. The long tail is the thing to notice: the rounder, short-tailed animals people also see in fields are often voles, which are cousins, not the same animal. Fieldmice eat seeds and plants, and they are shy. The word is simply a mouse of the field. It is an ordinary nature word, not a poetic one, though children’s books love it.