The term 'HORSE'S SOUND' can be solved by two crossword solutions: NEIGH (5 letters) and WHINNY (6 letters). Horses make a variety of sounds, but the two most common are neighing and whinnying. Neighing is a sound made by horses to communicate with other horses, and it is usually a short, loud sound. Whinnying is a sound made by horses to communicate with humans, and it is usually a longer, softer sound.