\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.