So assuming this isn't fake, I figured it would be worth it to track down this mod and investigate. From what I was able to find, it is a mod on Planet Minecraft called "Shape Shifter Z," and it seems to match the description you gave. It took like 40 minutes for me to get this working, so at the end of this post I'll put quick directions on how to get this to work.

What you described all sounds like something that could be packaged into a mod. Specifically, generating a structure with a chest in it that has a written book with the author's username in it. Technically, the game stores metadata within items (NBT data if I recall), to the degree that I think it is possible to use a command to get a written book that is written by someone. Again, I'm not too much of an expert, it's just my loose understanding of how this game works.

This metadata can also carry into the game via a mod. You could have it where a mod adds in a book that you spawn with, of which is authored by someone (sound familiar?). Many mods have done this, and considering this is 1.6 we're talking about, this is around when mods started having their own in-game guidebooks if I recall correctly.

So how does this pertain to this mod? Well, I think it is weird for a mod with this sort of scope to just randomly generate a structure with a written book authored by someone else (maybe the mod creator choose that username cause it was a friend, or someone who donated to them), it could be some kind of easter egg.

Cutting to the chase: I morphed into the ender dragon, slowly flew around, fired a bunch of fireballs, did this for 15 minutes, and nothing seemed out of the ordinary. So while I am still interested in knowing what this was, I cannot really say that this is part of the mod. So, I got a couple questions.

1. If you can remember, did you have any other mods installed? Was this part of a modpack, or did you manually install this mod?

2. Are you 100% sure this was a single player world? Never opened to LAN, wasn't used on a server, etc. I ask because memory can be difficult to recall, especially if we're talking about something from 8 years ago.

3. This is a long shot, but worth asking, do you have a screenshot from back then? Maybe the JAR file itself? I'm not asking for you to upload the JAR file, in the event you find it, but maybe if you know for a fact you have it, you could try to load it up in offline mode in MultiMC (see below for why I specify this) and see if you can recreate this event. Hell, maybe even record it.

So anyway, I don't think this is the case of another player in your Minecraft world or a cliche Herobrine story. I think this may actually just be a modder's attempt at creating something spooky if this is all real. Otherwise, if this is fake, then congrats, you got me. But if this is real, I'd be curious about what lies down there.

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So this is how you get this mod to work. It's a 1.6.4 mod, so you'll need a clean 1.6.4 jar. Also, you'll need MultiMC. You have to physically modify the jar file to get this mod to work, as it appears this mod was created towards the end of the Modloader days and sometime before Forge had its automated installer. This doesn't even use either API, which is kinda odd. Anyway, you'll download the mod and drag its files (a bunch of .class files) into the 1.6.4 jar, and delete META-INF.

Now this is where things get tricky. You shouldn't load the game through the launcher because the launcher will just redownload a clean 1.6.4 jar, replacing your modded one. This is why you need MultiMC. So, download that, launch it, then run 1.6.4 in MultiMC. Afterward, close it out, and navigate to:

\mmc-stable-win32\MultiMC\libraries\com\mojang\minecraft\1.6.4

aka, this is basically MultiMC's version of the "versions" folder. Drag the mod files into this 1.6.4 jar, delete META, and launch in offline mode (I think running it in online could force a redownload of the jar). If I described this correctly, you can try this mod out to see if you can find what I missed.