In June 1997, the first novel in the Harry Potter series was released, causing a worldwide phenomenon in anything related to it. With all of the theme parties, costume contests and release parties, it gets harder to come up with unique Harry Potter costume ideas.
Aragog, Hagrid's Spider
You can dress as Aragog -- Hagrid's large, blind, pet spider who died after a long life crawling around Hogwarts. When Tom Riddle released Slytherin's monster -- the Basilisk -- upon the wizardry school, Aragog was accused of doing the damage and thought to be the monster. To dress up as Aragog, dress as a large, black spider with eight legs. This costume is a great accompaniment to almost any character, especially Hagrid.
In "Harry Potter and the Goblet of Fire," Harry sits in a bath, trying to decipher a series of clues about the tasks of the Triwizard Tournament -- many of which he is given clues about from the Mermaid in the painting in the Perfect's bathroom. Dress as the mermaid by wearing a fake mermaid fin and a swimming costume top. Pile hair on the head and attach it up and apply glamorous make-up to play up features. Because the mermaid is in a painting, consider constructing a fake frame to insinuate that this costume is the walking painting.
Mr. Ollivander is the wand store owner and constructor of both the wand that is Harry's and the wand that is Lord Voldemort's. To dress as Mr. Ollivander, wear a suit that's maybe a size too large with a cape and white-collared shirt. Temporarily colour your hair white and style it to stick up and out, similar to Albert Einstein's. Wear a short, black scarf around your neck and tie it loosely. Go one step further by carrying a few homemade wands, offering them to other characters -- helping them choose their wand.
Tom, the Bartender
Tom is the innkeeper and bartender of The Leaky Cauldron, a pub and inn for wizards located near Charring Cross that serves as a gateway between the Muggle world and wizard world. To dress as Tom, put on a cap that helps create the look of a shaved head or head with no hair. Attach pointed ears and long fingernails. Tom has a hunchback and wears all black. This is a great costume for any party that requires a spirited bartender.