The Ooser seems to have been a figure to frighten. The definition given in an 1886 glossary by W. Barnes describes it a "A mask with grim jaws, put on with a cow's skin to frighten folk". In The Return of the Native, Thomas Hardy has a character ask "What have made you so down? Have you seen a [sic] ooser?"
The Ooser was owned by a family from Dorset (so it's sometimes called the Dorset Ooser). They left it behind when they moved to Somerset, in the care of a coachman. It disappeared in the 1930s, apparently after the coachman sold it. A replica is held in the Dorset County Museum.