Hidden in a stone
An Antamonstrum is a vaporous creature that acts essentially as a guard dog; shockingly fierce. Lying dormant in a cluster of purple crystal until disturbed, it viciously attacks intruders and apparently leaves no trace of any unfortunate enough to be enveloped in its foggy form.
Released
Stricklander used one to guard his office, and later to make Principal Levit "disappear" so he could take Levit's job. You cannot see the scene, but you can hear the screames of Principal Levit and the laughter of Stricklander.