Apparently -
Mottistone means ‘speaker’s stone’ in Old English. The Druids are supposed to have sacrificed white bulls beside the longstone (white cattle with red ears belonged to the fairies or came from the Celtic underworld, so perhaps that is relevant). The two stones here have been interpreted as male and female. Perhaps it’s more likely they are the remains of a destroyed burial chamber with more stones.
(folklore found in the Reader’s Digest ‘Folklore Myths and Legends of Britain’)