Snorgar (the Reaper pokemon): this and abnormal fusion only occurs when a Gengar attempts to manipulate the mind of a starving Snorlax. The insatiable hunger of the consuming pokemon's spirit then swallows the Gengar. Their bodies soon fuse causing the creation of a normal ghost type.
Snorgar, in its rare siting, is only seen at sites of disaster and battles, consuming the fallen souls of the warriors and victims. It is said is some regions that seeing this pokemon before any event is a dire omen.