A fan transformation of Bowser based off of Peachette from New Super Mario Bros. U Deluxe.
Just as Toadette in that game uses the Super Crown to turn into a Princess Peach-lookalike, Bowser uses the crown in the same way to transform himself into a variant of Peach, with differences being a black, strapless dress (sometimes interpreted as a black, strapless leotard with an open skirt), spiked bracelets, spiked armlets, a black spiked collar, sharp fingernails and sharp teeth, along with Bowser's spiked turtle shell and horns. Where visible, she also has Bowser's spiked tail and a set of blue earrings on pointy ears.
Three major variants of Bowsette exist: Fair skinned blonde, fair skinned redhead and dark skinned redhead. Which one shows up in a given image is usually dependent on artist preference.
In Japan, this character is known as クッパ姫 (Koopa-hime, lit. "Princess Bowser"). Other names associated with this character include Bowserette, Peacher, Princess Koopa, Peach Bowser, Princess Bowser, and... Beach.
Bowsette has officially been labelled as non-canon, as the New Super Mario Bros. U Deluxe official website claims that the Super Crown can only be used by Toadette.