The modding scene for Zelda: Breath of the Wild has exploded recently, and this showdown between Mario and Bowser is no exception. Trollsquad57 has actually put together several mods into one package (LynardKiller's Bowser, WilianZilz's Mario, and BeardBear's music mod) to create one epic showdown between the Koopa King and Fire Flower Mario, and for the most part, it works.
There are some concessions for movesets as Mario is basically operating as Link and looks a bit off, but the sheer quality of the Bowser model is on point; who is based off of the troll-like Hinox shell. While some might find this wacky, Mario has a long history of using weapons in combat (particularly in the Mario RPG/Mario & Luigi series), and I can definitely see an official Mario mini-quest Easter egg working out in a Zelda game.
via destructoid https://www.destructoid.com/mario-and-bowser-invade-zelda-breath-of-the-wild-in-this-clever-mod-506745.phtml