Chaotic

Chaotic is an attribute that, among other things, makes a creature extremely susceptible to silver weapons.

Chaotic monsters

Mimics, shapeshifters, all ugly things, abominations large and small, killer klowns, Tiamat;
A creature having a mutating attack or a chaotic attack (as opposed to simply having a chaos-branded weapon, like Crazy Yiuf);
A creature having an intrinsic chaotic spell, such as Mnoleg (as opposed to simply being a spellcaster with a chaotic spell, such as Kirke).