Any of group of tracheophyte plants belonging to the genus Equisetum. Horsetails have a perennial creeping rhizome supporting erect jointed stems bearing whorls of thin leaves. Spores are produced by terminal conelike structures. The horsetails and their extinct relatives have a fossil record extending back to the Palaeozoic with their greatest development in the Carboniferous period, when giant tree forms were the dominant vegetation with the Lycopodiophyta. Horsetails were formerly placed in a separate phylum (variously called Equisetophyta, Sphenophyta, or Arthrophyta), but recent molecular genetic evidence places them in a clade, the monilophytes, along with the whisk ferns and true ferns.