So some of you may be wondering things about me, like why I'm so purple, what things I like, and why I'm so darn big down there.
A select few of you may wonder where I came from.
And where did I go?
I am in fact, Cotton Eye Joe- no, no, just a big-dicked herm.

But! For those that would like some form of backstory for this far-too-big-for-its-own-good-horsecock-with-a-dragon-attached (I need to work on a better title..), I have a semi-solution; Far too many hyphens for one paragraph.
Oh yea, and a bit of backstory.
I hope you enjoy a little bit of story, and yes Timmy you may go to the bathroom, Bob please share your joke with the class, alright SHUDDUP!

*ahem*

Since he first joined magic school, Vanthrys always showed potential. He didn't really do much with it, but he could have ended up being so much more.
Perhaps he did, in this case.
He was frequently the 'victim' of several magic experiments, mostly harmless, but he was used for target practice a lot, never getting any help from the tutors besides the usual "Just try not to react". Pretty hard not to, when there's a meter-wide fireball engulfing your head.
Anyway, the main interest happened in the later years; more senior times, still the same experiments. This time, though, they were trying out various shape-shifting spells and, since you generally don't experiment on yourself, guess who got turned into various different animals of all range of sizes? It's a wonder Van never got morph-sickness.
Carry on through the years and you get to the inevitable "Is there a spell to make your dick big?" Guinea pig, step up to the table.
This time was with the few friends he had at the time, two other guys and two girls (every heard of being a fifth wheel?). Being sort of used to being experimented on, Vanthrys agreed to be paid for "magical advancement training" at the hands of the little circle. Paws, in the one case of a feral.
This is when he became so big. He used to be the typical, if small, dragon; Being around 9ft tall as an anthro, he had twin tapered, ridged lengths about 9 inches long each, and internal testes; his hemipenes got fused together, combining their thickness and adding length to just over a foot long, and got stuck in the shape of his most recent transformation; a horse. He kept his knot, but his balls were also transported into a sack. and doubled, and grew. And grew. And grew... Vanthrys was stuck cumming in the backroom for about 10 minutes before the janitor came to see where the flood of thick liquid was coming from, and got carried to his room to recover.
Cue another such session a few days later, this time devised by one of the females; the feral, to be exact. She wanted to know what effect a more feminine-oriented spell would do to Vanthrys, being so susceptible to transformation already. He wound up with becoming a hermaphrodite, being given a set of inch-and-a-half thick equine outer lips, very stretchy and almost rubbery yet soft, and the production from his balls found its way into giving him what seemed to be a swimming pool's worth of juice.
Also cue the janitor again, also 10 minutes later.
Fast-forward to the last year, 14 years of failing to stifle throbbing equine boners at every tease, and an inexplicable habit to check every room for where a drainage point would be. This was the big one, the win or lose, the yay or nay; Examination.
Except, something went wrong.
One of the younger dragons decided he didn't like the school, and skipped as many classes as he could, in order to study on his own; for a spell to destroy the place, a forbidden 'dark bomb'. And he succeeded. On that fateful day, Vanthrys' body was obliterated along with the school.
However, all that transformation had anchored his body into some sort of morphic field, and only the energy from that massive magical 'nuke', for want of a better word, had managed to actually settle it. Vanthrys reformed, surrounded by rubble, smoke, and debris.
His body was no longer the chitin and flesh of a scaled drake, nor the hide or fur of a feral, highland, or hybrid dragon. He was then, and then on to now, a creature made from solidified dark energy. Because of this, he is nearly impervious to physical harm, but is very vulnerable to heat and light; even natural sunlight is a mild irritant to him now, and any highly bright light will actually force him to disperse. Not painlessly, either.
However, that morphic field containing his body has him inflicted with the currently only known case of Morphic Hyperstability, recently coined as Vanthrys' Syndrome™. This grants him a massive resistance to any and all transformation sources, from himself or otherwise, and any that do somehow affect him are severely weakened, and extremely temporary, the current longest having lasted only a day before being reverted. Also not painless.

Journal is actually full now; if you have any questions, I'll try my best to answer them