The Collaborative International Dictionary of English v.0.48:

Horsetail \Horse"tail`\, n.
   1. (Bot.) A leafless plant, with hollow and rushlike stems.
      It is of the genus Equisetum, and is allied to the
      ferns. See Illust. of Equisetum.
      [1913 Webster]

   2. A Turkish standard, denoting rank.
      [1913 Webster]

   Note: Commanders are distinguished by the number of
         horsetails carried before them. Thus, the sultan has
         seven, the grand vizier five, and the pashas three,
         two, or one.
         [1913 Webster]

   Shrubby horsetail. (Bot.) See Joint-fir.
      [1913 Webster]

WordNet (r) 3.0 (2006):

horsetail
    n 1: perennial rushlike flowerless herbs with jointed hollow
         stems and narrow toothlike leaves that spread by creeping
         rhizomes; tend to become weedy; common in northern
         hemisphere; some in Africa and South America