There are two types. Field Horsettails and Marsh Horsetails.
These have been considered a fern, but many think of them as a distinctive group in their own right. As Milne Publishing explain : The genus traditionally had been put in a phylum all by itself but more recent treatments lump the horsetails in the fern phylum (Pterophyta), usually putting them in an order (Equisetales) distinct from other fern groups. When treated this way, the orders of the Pterophyta diverged in the late Paleozoic era. For ferns see index.asp?pageid=731954
Wiki tells us that they are only living genus in Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds.
Equisetum is a "living fossil", the only living genus of the entire subclass Equisetidae, which for over 100 million years was much more diverse and dominated the understorey of late Paleozoic forests.
Nature Scot tells us that these ancient plants have changed little since they covered much of the land before the age of the dinosaurs. In that period, some horsetails were as tall as our modern trees. Today, Mexico still has horsetails that reach heights of nearly 8m. Stems 10 to 60 cm tall and 1 to 3 mm wide erect or decumbent, green, slightly rough, with few (only 4 to 8), rounded vertical ridges and prominent unbranched branches in irregular whorls the sheaths loose, grey green with erect black white edged, one ribbed teeth. Cone terminal on main stem (sometimes on side shots) 2 to 3.5 cm.
Long ago, before people had access to scouring pads, the rough stems of some horsetails were used to clean pots and pans. Rough horsetail (Equisetum hyemale), sometimes known as Dutch rush, was particularly popular for cleaning metal.
A general Google search brings up mainly features on how to rid your garden of them. Many consider them a weed, but, as with any plant in the "wrong" place, they can be problematic. They can become invasive. In the "right" place though they can be extremely beautiful and intriguing as they progress in spring through their stages of primitive looking reptilian stalk to ancient spiky stages and from there to soft fern-like, or should we say horsetail like growth.
FIELD HORSETAILS : Equisetum arvense
Nature Scot website tells us that Scotland’s largest species is the great horsetail (Equisetum telmateia), which grows up to 2m tall. It is particularly striking when its spore-producing ‘cones’ appear in early spring.Locally though we only find small examples and these are most probably escapees from gardens. They have been seen in backroads of Alexandria rather than in the wild. Another location is the upper reaches of the River Leven where it grows almost in the water on the banks near Balloch Bridge. This appears to be the field horsetail and not its marsh cousin described below. This one has fine fronds when young while the latter has quite bold stalks.
Horsetails along the upper Leven in May. un like their heavier marsh cousins, these are soft and easily lean over. Look closely and you will see the branches starting to grow off the stems.
A soft horse hair-like clump of horsetails on the upper Leven.
If you look carefully you will notice drops of water at various points at the end of a stalk or its branches. Their ability to draw u water through capilary action is very evident.
This ability to draw up water is really fascinating. While this photo and the next was taken after a light rain, those preceding it were not.
MARSH HORSETAILS : Equisetum palustre
Many ponds are very intriguing places. A whole damp exosystem in their own rights. From tadpoles to damselfies to horsetails. No we are not suggesting that horses or their tails occupy our ponds. What we are looking at here are the plants called horsetails. Rising out of the wetness in May are stalks, almost asparagus looking.of course they change as they grow, sprouting hair-like fronds that give the plant its name.
New marsh horsetails in the wetlands of the RSPB Lomond reserve.
These in a small area of bog in the Auchnacraig Woodland Park have started to sprout their side fronds.
A stand of marsh horsetail in June when the water of the Leven was a seasonal low.
A close up of the horsetail heads.
MILNE PUBLISHING : https://milnepublishing.geneseo.edu/botany/chapter/equisetum/#:~:text=The%20genus%20traditionally%20had%20been%20put%20in%20a,the%20Pterophyta%20diverged%20in%20the%20late%20Paleozoic%20era.
NATURE SPOT : https://www.naturespot.org.uk/species/marsh-horsetail