Are the Enderman really slaves to the Ender Dragon? I was researching them and I read that they had been subjugated to slavery under the ender dragon and that they are virtually insane.
There is no official "lore" to the Minecraft game at this point, only theories. The official Minecraft wiki has entries for the game(s), but none contain any information; opting to delete the entries instead.
Delete because its fan-fiction and non-canon
This is because there is no information about the in-game monsters, creatures, etc, beyond their technical aspects.
Basically, as far as I am aware, the only "official lore" about the game is things like the fact that the Creeper only came about because of a code error in the generation of the pig.
Anything beyond this, while exciting is entirely theory, which is not something we handle here on Arqade.