A large, strong horse used to pull heavy loads: a cart, a wagon, or a plough. These horses have thick bodies and calm strength; they are not built to race or to look fancy. In Britain the word is often written as one word, carthorse; cart horse as two words is also fine. Today machines do most of this work, so the word feels a little old — farm, history, picture books. People sometimes use it, a bit unkindly, of a person who is strong but slow. The kind, usual meaning is still the animal.