Mummy is an ability exclusive to Cofagrigus and Yamask. When a Pokémon makes physical contact with a Pokémon with the Mummy ability, they lose their current ability and their ability becomes Mummy until it is switched out. This ability seems useless at first but can be useful to Pokémon with a hindering ability. An example could be Slaking with its Truant ability, which hinders Slaking from attacking consecutively. If Slaking then attacks a Cofagrigus or any opponent that has Mummy, its Truant ability is swapped for the Mummy ability until the end of the battle, or until it is switched out.

Pokémon

Pokédex	Pokémon	Sprite	Type	Obtained
style="vertical-align: top; text-align: center;#562	style="vertical-align: top; text-align: center;Yamask	style="vertical-align: top; text-align: center;	style="vertical-align: top; text-align: center;	style="vertical-align: top; text-align: center;Natural
style="vertical-align: top; text-align: center;#563	style="vertical-align: top; text-align: center;Cofagrigus	style="vertical-align: top; text-align: center;	style="vertical-align: top; text-align: center;	style="vertical-align: top; text-align: center;Natural