Definition of great horsetail in US English:

great horsetail

noun

  • (In early use) any of several large horsetails of the genus Equisetum; (in later use) specifically a large horsetail, E. telmateia (formerly called E. majus), of damp soils in Eurasia, North Africa, and western North America.

Origin

Late 16th century; earliest use found in Henry Lyte (1529–1607), botanist and antiquary.