"I understand Disneyland's Haunted Mansion is now open to humans."
"Humans! Wow, that's scary!"
—A reporter and Willie the Wisp
Willie is a spirit who was once an Olympic hide-and-seek champion. In 1969, he was interviewed in the Disneyland Haunted Mansion by a reporter, who asked what it's like in the Mansion. Willie replied, "Beautiful, man! Kinda weird, wild and freaky!" The reporter also interviewed Granny Ghoul and Phineas Pock. Due to Willie's love of hide & seek it can be assumed that during his afterlife, Willie became one of the Pop-Up Ghosts.
Willie's name is a play on "Will-o'-the-Wisp", a spirit from European folklore who is a glowing light that inhabits forests and marshes where it lures travellers to an early grave. While the story changes based on location, the origins of "Will o the Wisp" specifically is said to be the soul of an evil blacksmith named Will who was cursed to wander the Earth with only a lantern for eternity. Other European cultures identify Wisps as being inhuman spirits similar to Faeries or Demons which lure humans away in an attempt to cause mischief.
In reality, Wisps were used as a supernatural explanation for "Ignus Fatuus", a naturally occurring chemical phenomenon caused the oxidation of phospine, diphosphane and methane. This chemical reaction would happen in wetlands due to a heavy abundance of natural chemicals and decomposition, resulting in glowing and gas smells. Wisps leading travellers to their death was because typically "Wisps" would appear over water and only be visible at night, causing travellers to unknowingly follow the lights to a watery grave.
It can be assumed that Willie is not THE Will o' the Wisp from folklore but his being identified as A wisp can mean that he is one of many Wisps.