SonMario

SonMario is the crossover slash ship between Mario from the Mario series and Sonic from the Sonic the Hedgehog series (and by extension, from the Super Smash Bros. series).

Canon
At 1991, Sonic was created to provide Sega with a mascot to rival Nintendo's flagship character Mario. Nintendo and Sega were business rivals throughout the 1990s, though mostly during the competition of the Super Nintendo Entertainment System and the Sega Genesis / Mega Drive. As such, Nintendo and Sega often attempted to put each other down in their games. Their rivalry lasted until 2001 when Sega left the console manufacturing market with the discontinuation of their last console, the Dreamcast (in America and Europe) and became a third-party game developer to their former market rivals, including Nintendo.

Mario & Sonic series
"Mario and Sonic have been respectful rivals since the early days of video games. In fact, for a long time they have been discussing the possibility of one day competing against each other. Now that they have been given the perfect opportunity to meet at the Olympic Games, we may finally learn who is actually faster, Mario or Sonic?"

- Shigeru Miyamoto, creator of the Mario series.

Mario & Sonic at the Olympic Games is the first official crossover between the world's two most popular video game characters.

Fanon
Although its popularity is overshadowed by popular rival ships such as Mareach and Sonamy, SonMario remains existing since the 1990s, that even it can be considered one of the earliest ships in Sonic's fandom thanks to fans creating fan art of them together on published video game magazines of that time. The popularity of this ship has increased considerably in 2007 thanks to Nintendo and Sega's announcement of the Mario & Sonic series, and therefore, the pairing become common in both fandoms.

Trivia

 * The ship is named Somari incorrectly, as this is the name used for an unlicensed port of Sonic the Hedgehog (1991) for Famicom.
 * For Mario's 20th anniversary, 2D artwork of Sonic wearing Mario's hat was made on Sonic Channel.
 * In the popular children's show Oha Suta which broadcasts at morning on Japanese television station TV Tokyo, Mario and Sonic were invited together to cheer to celebrate Sonic Adventure 2: Battle's launch and to promote Sonic Advance for Game Boy Advance.
 * Mario was invited to the Japanese Sonic the Hedgehog's 25th Anniversary Party to promote Mario & Sonic at the Rio 2016 Olympic Games's launch and to celebrate his rival's birthday.
 * At Fund The Charity Room 2016, Jason Griffith —Sonic's former English voice actor— has flirted with Mario (who was performed by YouTuber SomecallmeJohnny at the moment) jokingly in his Sonic character.
 * The official Sonic the Hedgehog Twitter account compared twice Mario's moustache to Eggman's.