Mare's-tail \Mare's"-tail`\, n. 1. A long streaky cloud, spreading out like a horse's tail, and believed to indicate rain; a cirrus cloud. See {Cloud}. Mackerel sky and mare's-tails Make tall ships carry low sails. --Old Rhyme. 2. (Bot.) An aquatic plant of the genus {Hippuris} ({H. vulgaris}), having narrow leaves in whorls.