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.
- Supportive or chained mobs respond to any attack on a nearby mob. The Nimble Worm Piranha you encountered on Trainee Island would react to support/assist a nearby Worm Piranha or Boss Worm Piranha if you attacked either of these.
- On the other hand the Worm Piranha would not respond to support a Nimble Worm Piranha if you attacked the Nimble Worm Piranha. The Worm Piranha is a passive mob. (... unless it is chained to another mob!) There is not always a visible sign that reveals when one mob is chained to another. Red Drill is a field boss mob that spawns only after 4 or 5 Red Drill Guard mobs have spawned. Clearly those guards mobs are supportive mobs, i.e. they are chained to Red Drill as well as to one another.
- Aggressive mobs are simply those which have a low threshold for aggro. You can be standing or sitting still and a nearby aggressive mob may very well attack you. The Nimble Worm Piranha and the Boss Worm Piranha are aggressive mobs. Aggressive mobs tend to also be Supportive mobs. Since Epic 5 aggressive mobs have been given red names. This makes movement through a field of mobs much easier than it used to be. Some old-school or veteran players might say that it makes the game too easy.
It is usually wise to pull the supportive mobs in an area before pulling the aggressive mobs and last of all, the passive mobs.