For a very incomplete example, you have sentry style mobs which run to activate an alarm and "bring help". You have mobs which run in fear and aggro any additional ones they path too near to.
In general most developers have noted that players hated this type of gameplay because it meant that one idiot could ruin an evening for 40+ other players in the dungeon because the 'train' would then clobber all the other players on the way back to their individual spawn points.
WoW also generally doesn't have zone lines that you can escape out of. The fleeing mechanic isn't something that's all that challenging to advanced players who know how to stun and slow. It actually makes encounters easier because the monsters stop doing damage at 20% health or whatever. Contemporary games have other ways of adding challenge.
The monster would run then find bigger monsters and all of them would come back to kill you. Often killing every low-level player in the area.