Different mobs react to threat differently. In this respect there are three categories of mob reactivity: * passive (e.g. the Worm Piranha is a passive mob) * aggressive (e.g. the Nimble Worm Piranha and the Boss Worm Piranha are aggressive) * the name of an aggressive mob appears in red font * supportive a.k.a. chained (e.g. the Nimble Worm Piranha will react to support/assist a nearby mob) There is no consistent correlation between mob level and mob reactivity.