Welcome! StackMob is a bukkit plugin that is used by many servers - big and small - to enhance the experience of their players by 'stacking' mobs. This is especially useful for monster grinders as there are usually many of the same type in one place.
Find out more on the SpigotMC page - https://www.spigotmc.org/resources/stackmob.29999/ Report issues on the issues tracker.