Notch first disclosed the existence of a new mob in development on his Google+ account in a screenshot. He gave no information other than it was "creepy".

The screenshot was immediately enhanced and posted onto Reddit, where a commenter suggested the name "slender man", a popular meme: Notch liked it and decided to use a variation of that name, "Enderman".

After days of speculation and idea generation on Reddit and elsewhere, Notch decided to use some of the more popular ideas to define what the mob did, and described the Endermen's behavior on his Twitter account:

• The Endermen are peaceful. If you look straight at them, they freeze and look at you. When you look away again, they run fast. And teleport. (Source)
• They run TOWARDS you, not away. So if you happen to look at one, do not look away. (Source)
• (they stop being peaceful if you look straight at them) (Source)

Later, Notch went into great detail about what he is trying to accomplish with the Endermen on his tumblog:

I wanted this to be a mob you only saw in the distance and a mob you’d be afraid of, but when I playtested it, it mostly felt like a regular zombie. There’s was a distinct mismatch between looking creepy and not actually playing creepy. When I made it move faster towards the player when attacking, and deal more damage, it got more difficult and I started respecting it, but it never felt creepy or scary.

So I thought some about what “creepy” actually is, and it’s more about trying to avoid something from happening than it is about actually having that thing happen. If you know something bad can happen if you do the wrong thing, you will start thinking about your actions, and that might make things more scary.

Yes

1. Spawn Endermen

   

2. Wait for one to pick up a block

   

3. Hit Enderman

4. Observe teleportation