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. [ 1913 Webster ]

Mackerel sky and mare's-tails
Make tall ships carry low sails. Old Rhyme. [ 1913 Webster ]

2. (Bot.) An aquatic plant of the genus Hippuris (Hippuris vulgaris), having narrow leaves in whorls. [ 1913 Webster ]