subdirectory_arrow_right Wario World (Game), Nintendo (Company), WarioWare (Collection)
Attachment was an official Nintendo website made as a hub for developers and publishers licensed to work on Nintendo hardware. The site used Wario as a mascot, something that may not seem strange as his profession is designing video games in the WarioWare series. However, the site was opened in 1997, predating WarioWare by multiple years, and also predates the Wario World game for GameCube. Instead of renaming the site to avoid confusion and using the URL for the game's US promotional site, the URL was used for the game's website, something that more than likely caused confusion for Wario fans. WarioWorld was closed in 2016, having recieved very few visual updates since the 1990s, and now redirects to a more modern and professionally designed Nintendo developer hub.
person Rocko & Heffer calendar_month December 18, 2023
Archive of a Supper Mario Broth post about WarioWorld:

Forum thread about Wario World's promotional website, showing a screenshot with the URL:
subdirectory_arrow_right WarioWare: Move It! (Game), WarioWare (Collection), Wario Land (Collection)
In the cutscenes for Kat & Ana's stage in WarioWare: Move It!, Cractus from Wario Land 4 appears. This is the first time a Wario Land character has appeared in the cutscenes of a WarioWare game.

In the Japanese version of Move It!, Cractus has a slightly different name from the Cractus that appeared in Wario Land 4.

The latter game's Cractus was called フラワナ, or Furawana, a pun on "flower" and「罠」(wana, trap). The former that appears in Move It! however is called フラワニ, or Furawani, a pun on "flower" and「鰐」(wani, crocodile). This name change could mean the character was renamed, or the Cractus in Move It! is meant to represent a different individual from the same species.
person Rocko & Heffer calendar_month November 17, 2023
Cractus in Move It:

Move It's English character scroll, confirming that Cractus is in the game:

Move It's Japanese character scroll:

Wario Land 4 Japanese version Cractus boss fight:
subdirectory_arrow_right Wario Land 4 (Game), Super Smash Bros. Brawl (Game)
Attachment Upon the release of Super Smash Bros. Brawl, many Wario fans were disappointed and shocked to see that his moveset had a powerful farting attack, something that seemed unprecedented for the character to fans in the US. In Japan, Wario's association with toilet humor in promotional material dates back to Wario Land II at least. The marketing for Wario Land 4 is arguably when the gross-out humor could be considered most prominent, with the game having golden poop stickers included in its manual, and the official Japanese website going into immense detail on the size and shape of Wario's fecal matter:

"Hey! Stop it with the dirty jokes! Is something I should say!! However…since it’s a good question I’m going to answer it! Last night, I put down 10 plates of liver sauteed with leek, 10 crapes, 10 plates of rice curry. My morning poop was very yellow, it was a good color and a big one too! It almost touched the seat of the toilet! I know! The smell is a mix of tsukemono [pickled vegetables], raw eggs, carassius sushi. Try smelling it!"

Up until Brawl's release, Wario had not been associated with vulgar humor that much within actual games, and the only glimpses US audiences would've received of Wario's slobbish side in marketing would have been in animated commercials where he was animated to have saliva dripping out of his mouth (something that could be easily disregarded given that Nintendo used gross-out gags to market most of their titles in the US at the time, even for less edgy characters such as Kirby and Yoshi) and - by an extreme stretch - a subplot in WarioWare: Touched! revolving around Wario's bad oral hygiene. Following Brawl, farting would be cemented as one of Wario's signature abilities in a variety of Mario games.
person Rocko & Heffer calendar_month November 16, 2023
In an interview, the game's director and designer Hiroji Kiyotake was asked what the idea behind Wario's creation was. He responded: "We imagined Wario as the Bluto to Mario’s Popeye. The truth is, we kind of came up with the idea of the name first, and everything else came after. Since he was a “warui” (bad) guy, he should be Wario. And we had the idea to flip the M upside down. To our surprise, the idea was a big hit with everyone on the team."
Attachment The Super Mario Land 2 official strategy guide contained an interview with game's director and designer Hiroji Kiyotake. In the interview, Kiyotake stated that Wario's favorite food is crepes.
Attachment Wario's in-universe origins have never been completely explored in the games, however there have been attempts to explore his back-story in various officially licensed comics.

The comic "Super Mario: Die Verwandlung" (which translates to Super Mario: The Transformation) shows Wario merging from Mario as a result of a procedure by Dr. Light to cure Mario from his pixelation.

The comic "Mario vs. Wario" shows that during youth, Mario and Wario, when playing together, would often result in Wario getting either hurt or humiliated, believing Mario to be the cause for his bad luck.

The second comic, "Warios Weihnachtsmärchen" (which translates to Wario's Christmas Tale), a parody of A Christmas Carol, shows a flashback of Wario and Mario as babies building sandcastles but because Wario was mean to everybody, Mario would receive all the attention. Also during a graduation ball, Wario wanted to dance with Peach, Princess Toadstool, however because Mario was much nicer to her she instead danced with him.

While all these comics were officially licensed, they have never been acknowledged by Nintendo. Wario has however appeared as a baby in Yoshi's Island DS.
The idea for WarioWare's fast action microgames originated from a Japanese game called Mario Artist: Polygon Studio for the Nintendo 64DD. The game featured a game mode called "Sound Bomber" which features many of the microgames that later made it into the WarioWare games.
It's easy to assume that Wario's name was based on turning the M in Mario's name upside down, but this isn't entirely the case. The Japanese word for 'bad' is 'warui', so Wario is actually a shortened version of 'Warui Mario' (Bad Mario). This is also the case for Waluigi (Bad Luigi), and works even better in Japan, where his name is pronounced 'Waruigi'.