A mare is a female horse, donkey, or zebra that is old enough to have babies. Usually, this means she's at least three or four years old. While people use mare specifically for adult females, they call younger female horses fillies and adult male horses stallions or, if neutered, geldings.

Mares have played crucial roles throughout history. They've pulled plows on farms, carried riders across continents, and raced in competitions. Because mares can have foals (baby horses), people who raise horses pay special attention to which mares they breed, hoping to produce fast racehorses, strong workhorses, or beautiful show horses. A mare carries her foal for about eleven months before giving birth.

The word appears in some common expressions. People sometimes describe a confusing situation as a mare's nest, meaning something that looks important but turns out to be a mess. And those dark patches you see on the moon? These maria are large, dark plains of solidified lava on the moon's surface.