The bone slime is a negatively-powered ooze which not only slays other creatures but then animates their bones by instinct later. The origins of this ooze are not as many believe, that more mundane oozes eat undead and therefore become creatures of undeath themselves. No; in actuality, bone slimes are born from the Negative Energy Plane itself. Every so often, the Negative Energy Plane, for lack of a better term, bursts. In this spot, an infinitesimally small hole appears on the Material Plane. Instead of sealing itself, the planar hole instead forms a scab. In this case, that scab is actually a bone slime. It has negative energy properties even though it is not undead.