Harry never actually left the cupboard under the stairs and four other theories.

We, Harry Potter [3] fans, are a creative bunch, and we have so many weird fanons [fan fiction canon] and insane theories that to list them all would take forever, but here are some of my favorites.

• The most bizarre I can think of, and my personal favorite, is that Harry never actually left the cupboard under the stairs and actually created the wizarding world as an escape from the trauma of his abusive childhood. Hogwarts wasn't real, but something that his mind created to deal with the pain he was in. There are also similar theories about him being in a coma, in which all the characters are either made up or fellow coma patients around him.
• There is also the theory that is used frequently in fanfiction; that Harry Potter is immortal, due to the terms that the prophecy laid down. "Either must die at the hand of the other." This may imply that Harry can only be killed by Voldemort, and vice versa. Once Voldemort is dead, there is nothing that can truly kill the Chosen One. Thus, Harry has achieved immortality, and can't be killed. Of course, this is introduced mostly with angst and the sorrow of outliving friends and family. Yay.
• And one of the strangest: Albus Dumbledore is actually a time-travelling Ron Weasley. There are parallels between the character: both red heads, both injured left legs, both love sweets. And he always seems to know what's going on: surely this could only be achieved through time travel. Perhaps Ron came back in time to fix the timeline, and ensure Voldemort's defeat? We know time travel is possible in the Harry Potter universe... Obviously, this is pretty crazy — but some people suspect it. Of course, there's the "Dumbledore [is] gay and Ron married Hermione" thing. But that's a minor flaw.
• Ginny dosed Harry with a love potion. I have actually written a fanfiction about this and there are signs of it. We know love potions are legal and widely sold — even by Ginny's brothers. We know Harry has a sudden burst of spontaneous feelings for the youngest Weasley and Ginny was obsessed with him — and Mrs Weasley admitted to using love potions when she was in school. Suspicious... but probably nonsense. And it's chilling when you think of how a love potion would be classified in the muggle world...
• And the most beautiful (and poetic) of all: that JK's seven books are each one of her Horcruxes. She put a piece of her soul in each and now she — and the series — will live forever.

— Quora user Molly Taylor [4]

Crookshanks is actually Lily Potter's cat.

• In The Deathly Hallows, when the trio are in Grimmauld Place, Harry reads a letter hand-written by Lily. In the letter, she mentions that they had a cat and that Harry almost ran over it with his toy broomstick. Harry even wonders if it survived the attack by Voldemort and if it was alive.
• In Prisoner of Azkaban, the owner at Magical Menagerie from where Hermione buys Crookshanks tell her that she had had Crookshanks for "quite some time" and that nobody ever wanted him. This could correspond to our earlier timeline where the cat disappeared after Voldemort's attack on the Potters' residence.
• Soon after Crookshanks is bought by Hermione, he quickly recognises Scabbers and the shaggy dog as Peter and Sirius respectively. The fan theory goes that Crookshanks recognized them both because they would have frequently visited Lily and James when he was with them.
• There is also a popular theory (no doubt started by Tumblr) which states that the Boa Constrictor that Harry helped escape in The Sorcerer's Stone is actually Nagini, who becomes Voldemort's final horcrux.

  Now, nowhere in the book does it say which type of snake Nagini is, which is probably what fuelled this fan theory. But it most definitely is false. Simply because Nagini is poisonous, clearly evidenced by the attack on Arthur Weasley. Now this crushes the theory quite soundly, as Boa Constrictors are not poisonous. So Nagini and the Boa Constrictor are not the same.

Nothing has been proved or denied by JKR, so we can only hope.

— Quora user Yedu Krishnan [5]

Harry Potter is an immortal.

Well, after reading the entire series twice and having spent a lot of my time with Potterheads, these are the five theories that I actually believe may be true.

• Harry Potter [6] is an immortal.

  Now, according to this theory, in her prediction (while a real trance) Professor Trelawney said, and I quote —

  "Either must die at the hand of the other for neither can live while the other survives.”

  Most interpreted through this that one had to kill the other (as Harry Killed Voldemort). But, this theory interprets the prediction as, One must and can only be killed by the other. This drew a conclusion that Harry defied his death by killing Voldemort.

• Ron is a time travelling Dumbledore.

  This theory has been popular since 2004. It states that the chess game in book 1 is the allegory of the series as a whole. Ron plays the knight and the king, just as Dumbledore did in the war. Both are described to be tall, thin with a loose nose, and have auburn hair (for Dumbuldore, it did before it went white). Also, the fact that they have similar qualities, like Bertie Bott's Every Flavored beans.

• Dursleys were under the effects of Horcrux.

  This theory stands for poor Dursleys. No doubt, they were so mean to Harry, but a part of the series justifies thee actions, as well. A Tumblr theory draws our attention to the last book of the series, where Ron goes horrible because of a five minutes exposure to the horcrux. This theory suggests that Dursleys were so evil because they were exposed to a horcrux (Harry Potter) for 10 years.

• Deathly Hallows is a symbolic of all books.

  This theory of Tumblr states, in the last fight scene of the book Hermoine and Ron are chased by a creature (just like the first book), then attacked by spiders (like second book), then a werewolf appears (the third book), then do the Dementors (like the fourth book), the Order of Phoenix does (fifth book), and finally they got to Snape, the Half Blood Prince (sixth book).

• J.K. Rowling in reality is Rita Skeeter.

  The theory argues that, Rita Skeeter was banished from living in magical world after it was discovered that so much of her works were fabricated. So, she then went to live with the Muggles, and fell upon difficult times, so [she] turned to writing in an effort to make a living for herself. This would mean, that the story of Harry Potter is [100] percent true. This all makes sense and fits to reality for crazy Potterheads, as they think no one has that big of an imagination.

Although, the last point may seem a bit stupid to many readers, but as a Potterhead I feel it is a great speculation. These were the theories that made me think and reread the books. I hope they instill the curiosity in you as well.

— Quora user Mrigank Sinha [7]