Kumamon is a yuru-kyara sensation.
A yuru-kyara is a character or mascot that is usually totally made up just for the purpose of being a mascot.
He shot to nationwide fame and can now be seen on toys, souvenirs and… pretty much everything, everywhere.
He was created in 2010 for a campaign called Kumamoto Surprise to draw tourists to Kumamoto after the new shinkansen line opened.
For some more info on other yuru-kyara check out Tofugu’s blog.