Definition
A "plug" can have several meanings, but when referring to horses, it's a slang term for an old, worn-out, or inferior horse. π΄ It suggests the horse is no longer useful for anything other than basic tasks or is of low quality. It can also mean a workhorse. Think of it as calling a car a "beater." The horse may get you from point A to point B, but don't expect it to win any races. It's a bit like calling your old laptop a relic, but it still runs.