MobMerge

Introduction
This plugin will merge mobs of the same type together. When a mob has been merged they will have a counter over their head telling how many mobs are merged together. Upon destroying this mob, the counter will decrease and another one will spawn in its place until it reaches zero.

Configuration

radius: 5
period: 10
limit: 10
mobs:
  - ZOMBIE
  - SKELETON
color: DARK_RED
excluded:
  - world_nether

Should all be self explanatory. Radius is radius in blocks to merge, period is number of seconds before merging mobs, colour is the colour to make their overhead nametag, and mobs is a list of mobs to merge.

List of colours: https://hub.spigotmc.org/javadocs/bukkit/org/bukkit/ChatColor.html
List of types: https://hub.spigotmc.org/javadocs/bukkit/org/bukkit/entity/EntityType.html

License
MobMerge is BSD licensed, although there is no source included at this time.

Other
I wrote this plugin by request some time ago (April 2015) and am now open sourcing it. It uses only Bukkit and hasn't had any changes since then.