Horsetails are primitive vascular plants descended from huge trees which lived in the Paleozoic era (600-375 million years ago) and were the product of the Carboniferous period (270 million years ago). Coal seams were derived from them. Botanically they lie between the Ferns and the Clubmosses. The Horsetails are split into two differing types:
Field Horsetail (Equisetum arvense), Great Horsetail (Equisetum telmateia) plus two other less frequent horsetails Wood Horsetail (Equisetum sylvaticum) and Shade Horsetail (Equisetum pratense) have fertile stems which are stouter and arise before the photosynthetic shoots ('leaves'). In these the fertile stem has a sporangia-bearing part at the top where the sporangia consist of sacs containing spores. The two stems do not grow at the same time. These sporangia-bearing stems are stout and white to a pale-brown colour (not green) and arise in early spring before the green shoots of the photosynthetic part which is green.

The other type of common horsetails (and their hybrids) have cone-bearing photosynthetic shoots where the cones are again at the summit but this time on a stem which bears leaves as well. These are green.

Horsetails have a high silica content which once made them useful for polishing metal and wood.

Some Horsetails are poisonous, such as Marsh Horsetail (Equisetum palustre) and contain toxic alkaloids such as those based upon Palustrine (a macro-cyclic spermidine polyamine not to be confused with the habitat 'palustrine') and also Nicotine. Some of these are toxic thiaminase-like compounds.

Horsetails unusually have a great many chromosomes, 2n=216 (meaning they have 216 different chromosomnes, repeated twice over - many more than humans where 2n=23 who (normally) have but 23 different chromosomes, repeated twice over. The 2n being the ploidy number, in this case diploid. Tetraploids are 4n, Hexaploids 6n, Octaploids 8n. Those with an odd number of sets, such as 3n, Triploid, are usually sterile and cannot produce seed because the chromosomes cannot totally pair up during duplication.