Marestail

Marestail (also mare's tail) may mean:

Plants
 Conyza canadensis, a plant also known as Canadian Horseweed and Canadian Fleabane
 Equisetum, a genus of plants also known as Horsetail and Pipeweed
 Hippuris, a genus of aquatic flowering plants

Waterfalls
 Grey Mare's Tail, near Moffat in Dumfriesshire, Scotland
 Grey Mare's Tail, Llanrwst in the Conwy valley in north Wales

Other
Mare's tail, a type of cirrus cloud formation.