Introduced in Pokémon: Black and White, Natural Harmonia Gropius, commonly referred to as N, is something of an enigma. Having the "heart of a Pokémon" and being able to speak with them because of it has always made this character stand apart from the rest. He was even raised by Pokémon at first, only to be found and groomed to become the leader of a world that separated Pokémon from their perceived human abusers by his surrogate father, Ghetsis. However, many believe that N was never a human at all, but rather is a Pokémon in disguise.
In the Pokémon world, there are many different Pokémon that possess equally varied abilities. Some can levitate, some can create hail or sandstorms from nothing, but others have even more peculiar abilities. One such ability is "Illusion," held exclusively by the two Pokémon Zorua and Zoroark, who coincidentally enough are Pokémon largely associated with N. N indeed had Zorua in the past, which many believe he released and evolved into a Zoroark that watches over him, but because players are supposedly given that very Zorua as a gift in Black 2 and White 2, this is impossible.
The theory states that the human N, as players and anime watchers know him, never actually existed. Instead, N was the Pokémon Zoroark the entire time. While this may seem a little crazy at first, there are many instances where Pokémon have pretended to be humans, especially Zoruas and Zoroarks. On multiple occasions in the games, the player meets "people" that turn out to be Zoruas or Zoroarks. In one instance with the latter, a man even tells the player about Zoroarks before giving them a TM -- right after they step away and turn into a Zoroark before running off. This proves that Zoroarks can speak human language, and even explains how N is capable of speaking to Pokémon.
There is even a moment in the games where the player goes out to find a Zoroark that howls their name in their sister's dream. Once the player arrives at the location from the dream, they actually do come across a Zoroark that runs inside the ruins of N's Castle. Curiously though, once inside, the only person there is N, without any sign of the Zoroark. While some might just assume that the Zoroark returned to N's Pokéball, that would be impossible due to the fact that he had released all his Pokémon aside from either Reshiram or Zekrom (depending on what version of the game is being played). With that being said, it seems the only possible conclusion is that N and the Zoroark are one and the same.
As strange as it might sound, this could be possible even though N is supposed to be a trainer. Mewtwo famously tried to become a Pokémon trainer in the very first film of the series. He kept his Pokémon in Pokéballs and even had them battle other trainers in an official-style match. While Mewtwo ultimately gave up on this fantasy, it does show that it's possible.
The biggest wrench in this theory is that it would be impossible for no one to find out. However, just because it's impossible doesn't necessarily mean it would really change anything. Ghetsis would undoubtedly know of Zoroark's illusion, and there's even a lot of evidence to say that he did. He continuously claimed that N had the "heart of a Pokémon" and frequently claimed he wasn't a real human. While players probably assumed that these were either metaphorical statements or insults respectively, it could be argued that they were quite literal statements. Moreover, considering Ghetsis was proven to be a master manipulator and a phony who didn't truly believe what he preached, it's easy to imagine him as just another human using and manipulating a Pokémon for his own benefit.
All in all, N's status as a human is quite the mystery. Given all the evidence as well as Zoroark's importance throughout the games of that generation, it's quite easy to imagine this theory being true. Even on a physical level, they share similarities, most notably their long hair and slender frames. In some ways, N being a Pokémon in disguise would actually make more sense than it would if he wasn't. One thing's for sure -- if N is a Pokémon, everyone should be glad that he never kisses Ash. That was weird, Latias. That was real weird.