Plessie has those horizontal strips on his back, right? What if one difference was that this version had vertical ones instead, and would be black in color. (Similar to how Yoob's shell was black.) As for other features, I'm not sure. Since Plessie hasn't been showcased in the games as a glutton at times like Yoshi, maybe his Shroob version would be like the opposite of Yoob, and be thinner?