The Haunted Mansion is Home to Nine Hundred & Ninety Nine Happy Haunting Ghost. The original concept for the Mansion was drawn by Walt's first Imagineers Harper Goff. His original Main Street sketch had a crooked street leading away from main street leading by a peaceful church and graveyard, with a run-down manor perched high on a hill that towered over main street.

The attraction however was not included in the 1955 opening but came later with the expansion of New Orleans square. The project was handed off to Ken Anderson to build on the original concept and Ken studied old New Orleans and old plantation homes and came up with a few ideas that Walt wasn't too fond of. He was often quoted as saying "We'll take care of the outside and let the ghosts take care of the inside."

The original concept for the attraction included a tale of tales of a ghostly sea captain who killed his nosy bride and then hanged himself, a mansion home to an unfortunate family, and a ghostly wedding party with previous Disney villains and spooks like Captain Hook, lonesome ghosts, and the headless horseman. Some of the Universal Monsters were even planned to appear.

On August 9, 1969, the Disneyland version of the attraction was completed, and remained essentially unchanged for years until in 2001 when the Haunted Mansion is closed down for remodeling to make room for a few of Tim Burtons friend from "Nightmare Before Christmas".

Apparently in 2006 some additional spookiness was added but I don't want to give it away. I would suggest reading the Wiki on the haunted Mansion as it is chalk full of classic Disney Imagineering stories. Happy haunting BWAHHAHAHAHAHAH.