"We have 999 happy haunts here, but there's room for a thousand. Any volunteers?"
—The Ghost Host to guests
The Happy Haunts are the nine-hundred and ninety-nine spirits haunting the Haunted Mansion.
Happy Haunts vary in appearance, from translucent people-like apparitions, to slightly rotten corpses. Nonetheless, they all enjoy their life in the Haunted Mansion, and they enjoy the company of the living, without frightening them too much.
Promotional Material Story
The original story for the ghosts in the haunted mansion was much more meta-textual than most to come after it.
Walt Disney and the Disneyland's Ghost Relations Department began collecting these ghosts to live in the Haunted Mansion. Disney successfully brought in 999 of them in time for the opening of the original ride on August 9, 1969.
Despite having died three years before (although some of them already haunted Gracey Manor before it was turned into a retirement home, most notably the Ghost Host, Madame Leota and the Black Widow Bride).