Infighting

Infighting refers to creatures that are meant to be on the same side engaging in combat with one another. This can lead to detrimental effects in progress in a player's dungeon.
In Dungeon Keeper, this can happen under two circumstances: when so-called "natural enemies" are in the same Lair, or when a creature gets sufficiently angry (e.g. when a Horned Reaper goes psycho). In Lushmeadow-on-Down, it is stated that overcoming the former will 'temper your mettle - whatever that means.' One plausible meaning of this is that it will improve your ability to cope with difficulties; naturally, creatures fighting amongst themselves can only add to your woes, and is something you could do without even at the best of times.
Infighting does not occur in Dungeon Keeper 2. Instead, creatures who dislike their company just get upset, though, as a result, may eventually rebel (defect to an enemy) or leave your dungeon. However, it seems that at some point infighting was going to be featured, for there is a Mentor line that's nearly word-for-word identical to its Dungeon Keeper counterpart.