Rereading The Hobbit at work, and had an insane thought when I got to Tolkien's painting of Smaug. The dialog is going to be off, I'm working from memory and this is just a quick sketch, as it were. If you haven't read The Hobbit, this is just going to horribly confuse you. This is nowhere near close to canon, I haven't fact checked names I've just used what seems good when my fingers hit the keys.
Bifur and Bombur scrambled up to get to the secret door, a couple of other dwarves helping. They stumbled over... a Man?
"What's a human doing here?" Bombur cried. "So far from Laketown!"
"I don't think he's from there," Fili said. He and Kili had been sent to help get the other two. "We'd better bring him along, it won't do for the dragon to kill and eat him."
Once the company was hidden in the back entrance to the Lonely Mountain, they examined the strange Man. He was dressed in robes, much like Gandalf's own, but in black. He didn't have a hat, or a staff, which seemed odd to them. Thorin woke him up carefully, a few of the others hiding weapons behind their backs just in case.
"Wha... where am I?" the Man groaned in the common tongue.
"You're inside the Lonely Mountain, near Dale," Thorin answered. "Thorin Oakenshield, at your service. And might I ask who you are?"
"Harry, Harry Potter," the Man answered back. He rubbed his eyes, squinting slightly at the dwarf as he produced a pair of spectacles such as a clerk might use. "Oh! And at your service too, I suppose. I guess you want to know why I'm here."
"Please," Bilbo said, keenly curious.
Harry blinked at the sight of someone even smaller than the dwarves before answering. "I'm... well, I was, now, a famous wizard in my homeland. I wanted to leave and find adventure, and a friend who acts more than he thinks gave a strange stone to me made of deep green jade, but it caught fire, and sent me here, somehow."
"You're neck deep in adventure now," Thorin said. "We're currently trying to think of a way to defeat the dragon Smaug, a wicked worm."
"Well, I've faced dragons before, I could have a go," Harry offered. Seeing them become suspicious, and remembering lessons in History of Magic, he added, "for a share of the reward, I mean."
"Remember that it's a dragon down there," Bilbo said. "If it looks like he's going to move, you need to run immediately. I think I got him quite angry when I stole a cup from him earlier."
"Oh, I have my ways," Harry said. "Anyone want to come with me?"
Before they left the small tunnel to enter the dragon's sleeping chamber, Harry stopped to brief Bilbo and Thorin (who were the only ones who came with Harry.)
"Alright, I'm going to try and defeat him mentally," Harry explained. "In his mind, I mean. Anything could happen, anything at all. I could catch fire, he could shatter, anything's possible when you're Legilimencing a completely different species with strong magic of it's own. So if things go really bad, you're to grab my body, run back, and wait for me to wake up."
That said, Harry drew his wand, strode up to the dragon's face and confidently shouted "Legilimens!" as a great eye started to crack itself.
Thorin and Bilbo watched anxiously. So far, the strange Wizard was holding a staring match with the wyrm, from what the two of them could tell. Sweat poured down Harry Potter's face, but it was as if a statue was being rained on. The dragon was similarly motionless.
With a great flash of light and crash, the two combatants collapsed, Smaug onto his hoard and Harry on his face in front of the dragon. Being a decent sort and a dwarf of his word, Thorin picked the Man up (for dwarves can carry a massive load indeed if the need arises) and ran with Bilbo for the back entrance tunnel.
When the man opened his eyes, it was clear that "Harry Potter" was not in residence.
"Who dares to steal from my hoard and attack me, Smaug?" the Man roared.
Gloin gave the Man a smart hit on the head, knocking him out. "You don't suppose?"
Bilbo sighed, pulling out his magic ring he'd taken from Gollum earlier. "I could go look."
"You're the burglar, it's your job," Bombur said.
Running down the tunnel as quietly as he could (which was completely silent to people like you and me), Bilbo saw a very confused dragon indeed.
"Bilbo?" the dragon called out in a deep voice. "Thorin? Er, this is Harry Potter in here, I think something has gone very, very wrong indeed..."
"What a horrid thing to happen!" Bilbo cried.
"It could have been worse," Harry said, a glint in his dragonish eye.
"What could have been worse?" Thorin asked.
"Smaug could have been a toad as big as a house that attacked, or maybe a giant dog, imagine being stuck in a body like that and not a very imperial looking dragon."