One theory is that Rubber Man is a human, and possibly one of the already introduced characters in disguise.

It could also be a ghost wearing the suit.
* Hugo: Hugo died in the house, and has yet to be seen, so it would make sense. He is also a womanizer, so it would explain why the rubber man had sex with Vivien.
* Doctor Charles Montgomery: Another candidate.
* Tate: Tate has been seen wearing the rubber suit. Actor Evan Peters has also said that he still has a "big secret" to reveal to Violet, even bigger than him being a school shooter and a ghost.^[1]
* Beauregard, because if we include other male ghosts, there's no reason to exclude him. Rubber Man never speaks.
* R. Franklin, because we don't know his final fate, and whether he died on property.
* The 4th child of Constance: She did tell Vivien (when she delivered the cupcake) that she had four children. So far, we have only been introduced to Addie, Tate and Beau.
* Sam Argento: Nikki's husband, they practiced S&M and they died inside the house, turning him into another candidate.
* The killer of Sam or Nikki Argento, as he murdered them with S&M objects
* Chad, he was the first to buy the suit to spice up his relationship with Patrick which was on the verge of falling apart; he wanted to start a family and have a child while his deteriorating relations with Patrick made it impossible to happen; Chad said himself that the house belongs to him and he acts extremely possesive about it;
* The undead Westfeild high kids: One or all of the undead Westfeild high kids could be the Rubber Man. The kids all have similar body builds and height similar to the Rubber Man. They also might be doing this because they have a grudge against Tate. They could have mistaken Patrick and Chad for Tate therefore they killed them.

The suit could also be sentient and/or can move all on its own without the help of a wearer.

Another theory is that Rubber Man is part of the house, or just a representation of it.

Such as The Devil, a demon, a vampire, or a constructed being.