A plant in the taxonomic genus Equisetum (horsetails); Equisetum arvense.
Equisetum, a fern ally
especially, common mare's tail (Hippuris vulgaris)
Erigeron canadensis (syn. Conyza canadensis), an annual weed
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.