Dreams and visions often reveal the latent meaning of social practices and institutions. This study is concerned with the relationship between nightmares and the institution of voodoo, the animistic religion of Haiti. In that system, the divinities manifest themselves by 'possession' of people whom they ride like horses. The language roots of the 'mare' in 'nightmare' are explored. Sexual, ghost, demon and horse connotations are found. Jones has postulated that the nightmare is closely related to the problems of incest and the primal scene. 'Possession in Haiti, viewed as a religious and socially recognized phenomenon, and as an important component of the voodoo ritual, may perhaps represent a ritual elaboration of the nightmare.'