Ender Golem

This mob would be created in the same way as an iron golem, except that the iron blocks would be replaced with obsidian. Unlike the existing golems, this creature would be hostile (and very dangerous). Additionally, the Ender Golem would only spawn when the blocks are placed by Endermen. My thought was that, since in the Overworld Endermen spawn in groups, every time a group spawned, there would be something like a 5% chance that they would begin a sequence of spawning an Ender Golem. In the first phase of this sequence, the Endermen would find and collect 4 blocks of obsidian. Once this has been found, they would place them in the proper T formation. The second Phase would be to find a pumpkin and place it on the T formation, thereby spawning the Ender Golem.

I would imagine that any Endermen that come within a certain proximity of the Ender Golem would stick with it and act something like a guard, becoming hostile at players without provocation. Also, the golem would have to drop lots of XP and a rare item. Which item, I'm not sure. It would probably make sense for a new item to be introduced.