The Collaborative International Dictionary of English v.0.48:

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.
      [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]