Currently only a handful of monsters can interact with each other in ways that do not involve healing. Here are some suggestions for possible monster interactions!

Gremlin Demos and Bombies: In the off chance of encountering these two monsters at the same time, demos would be able to throw bombies that got too close. The bombie would be killed (dropping its loot at the demo's feet) and its skull would be flung at a targetted player similar to a thrown pot. Getting hit by the bombie's head would deal similar damage to getting struck by a howlitzer head with the addition of the status the bombie would have otherwise inflicted.

Lichens and non-slime monsters: A small percentage of normal damage lichens (a likelihood which grows with the lichen's size) will target other monsters rather than players, as determined randomly by the game's code. Provoked monsters would fight back. The inference is that the lichen is hungry...

Menders and thwackers: Attacking a mender with gremlins thwackers nearby will cause all the thwackers within a certain radius to target the attacker.

Of course, these are only some examples of how monsters could interact. Give your own opinion on this idea and/or your own monster interaction concepts here!