Equisetum (pron.: /ˌɛkwɨˈsiːtəm/; horsetail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds.
Equisetum is a "living fossil" as it is the only living genus of the entire classEquisetopsida, which for over one hundred million years was much more diverse and dominated the understory of late Paleozoic forests. Some Equisetopsida were large trees reaching to 30 meters tall. The genus Calamites of the family Calamitaceae, for example, is abundant in coal deposits from the Carboniferous period.