They are classed as undead and are weak to the Holy element. The Unknowns attack by inflicting a myriad of status effects.
Each Unknown has its own set of moves, although the attacks Sap, which is an attack that can also inflict the Sap status, and Slimer, which inflicts Slow, are shared by a number of them. The rare #168 Unknown tries to use Delta Attack to deal damage and inflict Petrify, but fails due to a lack of MP. Beast Killers can be stolen from this enemy. The #169 Unknown uses a move called Possess, which instantly kills itself and removes a party member from battle, much like Sacrifice.
In the concept art for the Unknowns, Tetsuya Nomura names each of the creatures: the small, fungus-like Unknown is called "Deoxyryote", the large, drooling Unknown is called "Corrosive", and the worm-like Unknown is called "Doom".