Definition of horsetails:
(n) :
(botany) Any of various simple vascular plants, of the order Equisetales, that have hollow stems and produce spores.
(n) :
(literal) A tail of a horse.
(n) :
(anatomy) cauda equina, a bundle of nerve fibers.
(n) :
(botany) A mare's tail, a water plant in genus Hippuris.
(n) :
Any waterfall, the descending water of which maintains contact with bedrock most of the time.
(n) :
(military) A Turkish standard denoting rank.