Despite originating in Nintendo's Super Mario Bros. 2, the Shy Guys have gone to be better associated with the Yoshi's Island spin-off series where the creepy creatures play the role of Goomba to Yoshi's Mario.  Extending that line of thought will get you to the idea that perhaps the master antagonist of that trip to Subcon, Wart, should turn up in the Yoshi's Island world as well as this work of fan art from Michael Julius Peterson demonstrates.  If this image looks familiar, that's because it follows on from a piece of official Nintendo artwork used to promote Super Mario Bros. 2 (and, before that, Doki Doki Panic) but adds a few Yoshi characters into the mix (and a Yoshi-inspired pterodactyl which needs to be in an actual Yoshi's Island game immediately if not sooner).  If you need a refresher on that original promotional image, you'll find it below.  Feel free to compare and contrast.