Why do we call him Mr. X?

[R1ch4rd N1x0n]

Isn't his name T-00? Where the hell does Mr. X come from?

[Koala 4peace]

You need to ask Capcom about that one.

[cuss]

I think it might hark back to the OG, I think Umbrella (and Capcom) called Tyrants MrX.

[Charlie Foxtrot]

Over the years T-00 has been given a variety of nicknames in official media, though none are canon. One of the nicknames used in licensed material is "Mr. X", a name never used in Resident Evil 2 itself. It is possible this name was the result of a mistake on Capcom USA's part, as in multiple occasions it gave out early concept art of T-00 to licensees, which included one known as "Tyrant X" in Japan.
-Assuming this, the first instance of "Mr. X" was GameFan's Resident Evil 2 guide, which was published alongside the game though written in 1997.
-An action figure, also known as "Mr. X", was released in May 1998 by Toy Biz, though the name was already confirmed in an ad in the GameFan guide.
-The Pocket Books novel, Resident Evil: City of the Dead (released May 1999), has Claire Redfield pick "Mr. X" as a nickname from a list also including "Frankenstein's monster", "Terminator" and "Dr. Evil".
-"Mr. X" was used again in Resident Evil: The Official Comic Magazine #4 (published December 1998), where in this case it was the actual name of the series, with "Tyrant" instead only applying to identical clones of the Resident Evil foe.
-"Trenchy" and "Coated Bastard" (コート野郎) were used in the English and Japanese dubs of 2010's Resident Evil: The Darkside Chronicles
-"Mr. X" and "Trenchy" made another mention in 2020's Resident Evil: Resistance where they are nicknames given to a T-103 by Daniel Fabron. Both these names were included as in-jokes to the fans.

[cuss]

Tyrant X, interesting.

[whereismymind]

iirc it was a toy of him that coined the name, and everybody just kind of went with it, but i imagine the aforementioned guide is the correct answer

[DasaKamov]

Charlie Foxtrot nailed the gist of it - but I'll add that humans looooooove making up nicknames for scientific or military projects, such as "Warthog" for the US' famous ground-attack aircraft, or the "Messenger" for an Iranian main battle tank. ;)

"Mister X" is totally believable as a nickname for the Tyrant in the game-universe, since from a distance he would appear as a very large but mostly-normal human (hence the "Mister") but as he drew closer and his features became clear, it would be obvious that he is anything but normal and very much an unknown factor (hence the "X".)

[Alby]

Because he's gon' give it to ya

[bonjovi]

Cause Mr. T was already taken.

[John Warhammer]

Mr. X is often used when a title, or name, isn't known. Hence why he was labelled as such in this.

[AceLeoz]

I don't care what his name is but he sure as hell made me lose my crap while playing re2

[R1ch4rd N1x0n]

Originally posted by Dusty:

Mr. X is often used when a title, or name, isn't known. Hence why he was labelled as such in this.

But I thought that was John Doe?