mulo (Sg. m.), mule (Pl. m.) - "dead, ghost(s) of the dead";

Belief in the mulo is shared by many members of different Roma communities. A mulo can appear to settle accounts with someone, because s/he is missing something in the next world, because s/he disapproves of the behaviour of her/his survivors, but also to warn a dear survivor of a threat of danger. Usually a mulo strikes fear, but may also behave quite "normally" or represent a threat to living people.

Source and further reading:
ROMBASE © by Milena Hübschmannová 2002