Use hoarse for a rough or strained voice, and horse for the animal (or the gymnastics equipment). Think of the extra letters in horse spelling out a whole creature, while hoarse is just about how you sound: her voice was hoarse after the concert, not she rode a hoarse.
The core difference
Hoarse is an adjective about a strained voice; horse is a noun for the animal.
- hoarse — having a rough, strained voice: After the concert, her voice was hoarse from singing all night.
- horse — a large hoofed animal, or the gymnastics equipment: She rode her horse along the beach at sunrise.
- horse (verb) — to fool around, usually as "horse around": Stop horsing around and finish your homework.
How to tell them apart
A useful anchor: horse is a whole animal, and animals are things you can see and touch, so it is always a noun (or, in "horse around", a verb about behaviour). Hoarse only ever describes a voice, so if a sentence is about how someone sounds rather than what they are riding, it needs hoarse.
The confusion is purely a spelling one, since the two words are pronounced identically. It tends to catch writers out in phrases like "a hoarse whisper" or "a hoarse cheer from the crowd", where the unfamiliar spelling of hoarse loses out to the much more common word horse.
