"Horse" is an informal word that is used in both a narrow sense (sensu stricto) and in a broad sense (sensu lato).

Some people use "horse" in a narrow sense to indicate some (but not all) members of the modern domestic horse species Equus caballus (ek'-kwis ca-bal-lus). For example, some may correct others for referring to a "pony" (e.g. Shetland pony) as a "horse," though both belong to the same species.