horsetail vs mare's-tail

horsetail

noun
  • A mare's tail, a water plant in genus Hippuris. 

  • Any of various simple vascular plants, of the order Equisetales, that have hollow stems and produce spores. 

  • The tail of a horse. 

  • cauda equina, a bundle of nerve fibers. 

  • A Turkish standard denoting rank. 

  • Any waterfall, the descending water of which maintains contact with bedrock most of the time. 

  • A ponytail hairstyle. 

mare's-tail

noun
  • especially, common mare's tail (Hippuris vulgaris) 

  • Erigeron canadensis (syn. Conyza canadensis), an annual weed 

  • Equisetum, a fern ally 

  • A long streaky cloud, spreading out like a horse's tail, and believed to indicate rain; a cirrus cloud. 

  • Any aquatic plant of the genus Hippuris, having narrow leaves in whorls. 

How often have the words horsetail and mare's-tail occurred in a corpus of books? (source: Google Ngram Viewer )