In message , Sacha
quotes
I've been contacted by Mr Charles Bailey who points out that Horestail
is correctly applied to the weed growing on land whereas Marestail is
correctly applied to the weed growing in water.

Mare's tail, Hippuris vulgaris, is an aquatic flowering plant, with
reduced flowers (perhaps water-pollinated) belonging to family
Haloragaceae. Apart from the water-milfoils, which belong to the same
family, current opinion is that its nearest relatives in the British
Flora are the stonecrops (Sedum), pigmyweeds (Crassula) and navelwort
(Umbilicus).

Horsetail, sometimes called Mare's tail, Equisetum sp, are non-flowering
plants, and are probably modified ferns.
--
Stewart Robert Hinsley