A/N A random idea that popped into my head like a house elf a while ago. Contains Creature!Harry.
"AAHAHHHHHH!!!!! What is happening to me?!?!" Harry jumped up and down on his bed at 4 Privet Drive, scratching and twisting.
"SHUT UP FREAK!" Vernon bellowed from below, "We are laughing at the nerdy people playing Jeopardy. Now be quiet."
Harry continued to roll and twitch until he finally passed out. When he woke up, he sneezed. Magic flew out of his nose and hit the mirror. As it slowly dripped down, the mirror yawned, looked up and said "Goodmorning. Thanks for waking me up. Oh my, you should do something about that hair."
"Harry can't do anything about it," Harry snapped, then clasped his hands over his mouth.
"When did I start talking in the third person?"
He heard noise outside his door and all locks were being unbolted.
"Harry," Remus Lupin said, "I am to bring you to Headquarters."
The worried look on Lupins face didn't escape Harry.
"Alright, Professor Loopy sir," he nodded. "I am sorry! Professor Lupin."
Lupin sighed. "That's alright, Harry. Now we need to Apparate.."
They entered Headquarters, where Dumbledore sat gravely at the kitchen table, accompanied by Moody, Snape, Molly and Arthur Weasley. Uncharacteristically, there were dirty dishes in the sink. Harry immediately set to cleaning them.
"Harry," Dumbledore said softly, "Please come sit with us."
"Yes Headmaster Dumbly," Harry paled and clasped his hands over his mouth. "I'm so sorry."
"That's quite alright, my boy. I daresay I am rather fond of that name," The Headmaster twinkled slightly.
"Potter," Snape began, "I did some research into your line. I found evidence that your mother did not, in fact, hail from a Muggle line."
"She didn't?" Harry asked in surprise.
"No, Harry," Dumbledore continued gently, "Now, this may come as a shock to you, but it seems your mother wasn't even fully…human."
"What do you mean, sir?"
"Some lines of wizards have…bonded with creatures," Professor Lupin explained, "like de Delacours."
"With Veela," Harry nodded, "Am I a Veela? No, I'm not beautiful like Fleur and Gabrielle."
"Indeed," Snape sneered, although not nearly as evilly as he had used to, "it is quite uncommon for Muggles to bond to a creature, but it has happened. When the offspring is Muggle, this does not influence them much. However, when the offspring is magical…"
"Then the latent genes become active and create a…a throwback…." Lupin hesitated.
"It means that your aunt and cousin, not being magical, notice little effect from the genes," Dumbledore explained, "your cousin especially, since he is one more generation away from them. But your magic woke up those genes and merged you. Now that you are sixteen, they have become…active."
"I've never noticed anything," Harry said.
"You wouldn't, Potter. Not until now. The genes were dormant."
"Well, what sort of creature is it, then?" Harry asked impatiently, tapping the table. Magic flew from his fingers onto it and polished the surface to sparkling clean.
"We – found a…relative…who can explain it better," Dumbledore hedged.
Dobby popped in.
"Yous grandfather is being Dobby's brother, Harry Potter sir. You is being Dobby's nephew!"
Dumbledore looked apologetic, while Dobby nodded in satisfaction.
"It is being true, Harry Potter sir. You is being part House-Elf."
Harry thought back. "Is that why I always was so good at cleaning?"
A bright smile from Dobby affirmed this.
"I thought I was short for my age because my relatives starved me," Harry mused.
"No, Harry Potter sir. It is being your house elf blood."
"And Aunt Petunia's obsession with things being clean…"
"She's a Muggle, Potter. Her house-elf blood doesn't affect her much, but it could have some interesting side effects." Snape explained.
"It all fits!" Harry cried happily, "It's true! I AM a House Elf!"
Then he paled. "What do I look like now?"
Dumbledore conjured a full sized mirror silently, and Harry stared into it. He had shortened slightly, a few inches maybe and now stood at 5 foot. His ears had become pointy, his green eyes slightly wider, and his skin, formerly pale, now had a slight greenish tint to it, as if he was going to be sick.
"Yous be having your House Elf magic too, now, Potty," Dobby proclaimed, "yous need to learn to control it. I be teaching Potty."
"Thank you, Uncle Dobby," Harry smiled widely and bounced a little on the balls of his feet, "when can we start?"
"We's be starting right away," Dobby took Harry's hand and popped away, leaving some very bemused wizards in the kitchen.
"Potter," the snakeman hissed, "what are you doing?"
"Cleaning," Harry answered brightly.
The floor of the Dark Lord's throne room was now spotless and shiny.
"WHAT are you doing here, Potter? Besides cleaning my floor?"
"Well, Uncle Dobby and I thought you have been causing enough trouble. So Uncle Dobby and I be vanquishing yous."
The Dark Lord and assembled Death Eaters laughed.
"And how, boy, did you plan to do that?"
"House elf magic."
Voldemort laughed and stepped forward, only to slip up on the floor and fall on his backside. His Death Eaters, out of self preservation mostly, tried not to laugh.
Harry and Dobby bound a couple of sponges under their feet and sailed across the room easily.
"Look out, Uncle Dobby!"
"Dobby gots it," his Uncle assured him and bound the Death Eater trying to curse him.
"Yous got bad dark magic," Harry told Voldemort, "Yous be needing a good cleaning you do."
With that, Harry flicked his fingers and dark magic began to siphon out of Voldemort, who writhed in pain and screamed as the magic holding his body together was lost.
"Yous been a bad wizard," Dobby said in the middle of holding back Death Eaters, "Potty is being strong now. House elves be training him."
"The…Power…I knew…not," the dying Dark Lord gasped.
More and more magic seeped from him until his body fell apart in thousands of little scales.
"Yuck," Harry said, "I's be cleaning that."
With a single wave, the scales vanished.
"Uncle Dobby, are you finished?"
"Yes, we be going home now. Order will take care of those. Dobby wiped off the blood."
With that, Uncle and Nephew popped out of Riddle Mansion and back to the Hogwarts kitchens to do some celebratory baking.