- "Ah! Help me! I am not a monster! The spider's curse... It made me this way..."
- — Swamp Spider House Resident
The Swamp Spider House Resident is a character from The Legend of Zelda: Majora's Mask. Sometime prior to the events of the game, he entered the Swamp Spider House and found a man that gave him the Mask of Truth and a promise of a way to become rich. Soon after, he was transformed into a Skulltula by the Gold Skulltulas who reside there.
At one point, the man and his dog entered the Swamp Spider House and found a mysterious person. This person gave him the Mask of Truth, telling him that it contained instructions that would lead him to finding great treasure inside the mask itself. However, he was cursed when he put the mask on, as in-game dialogue explains. The only way to break the curse is to defeat the thirty Gold Skulltulas, but the man was unable to do it by himself. Instead, he waited for a hero, and, eventually, Link comes along. Link ultimately defeats the thirty Gold Skulltulas and lifts the curse. As thanks, the Swamp Spider House Resident gives him the Mask of Truth.
The Mask of Truth never turns its wearer into a monster by itself.
The man wears the mask because of greed. This is shown by both the story as to how he turned into a monster and, when Link picks up the dog in the same room as the man while wearing the Mask of Truth, he is told that his master wore the mask due to his greed. Clearly, when the man put the mask on, he was performing an act of greed.
In Hyrule, specifically Ocarina of Time, Link sees an entire family that is cursed due to its greed. The physical manifestation of the curse involved spiders dispersed throughout Hyrule and the family being turned into spiders. This curse being born from greed is shown to exist.
The location itself was known for spiders in the first place. Outside, Link can read a sign stating, "Fearful Spider House," clearly referring to the cave behind it. Given the fact that the dog is in good health and not dead due to starving, the cave was obviously referred to as the Fearful Spider House before the man went there. This place is tied to the curse, and it was not simply the place chosen for the Gold Skulltulas to roam free due to its close proximity to where the man put on the mask. Instead, it was simply "activated", for lack of a better term. The switch is performing an act of greed in or near it.
It is possible that the Swamp Spider House Resident's dog used to compete in Doggy Racing at the Romani Ranch. This is supported by the fact that the dog comments on how much it misses "the ranch" whenever Link picks up the dog while wearing the Mask of Truth after completing the Spider House. The dog's dialogue restarts every time Link resets time.