Yoob was a giant Shroob based on a Yoshi who appeared in Mario & Luigi: Partners in Time. He ate all of the Yoshis and put them in eggs. He then ate mischievous Baby Bowser, heroic Mario, ghostbusting Luigi, excited Baby Mario, hilarious-loving Baby Luigi, and the smart Toadbert. A boss, Sunnycide, is inside Yoob controlling him.
Yoob actually functions as a giant, moving, living factory. The Shroobs created this monster so that he can eat all of the smaller Yoshis and turn them into Yoobs as well. However, Yoob was shut down for good after Mario, Luigi, and the babies defeated Sunnycide. Yoob's eyes went blank and the rest were forced to make a daring escape through a rather unpleasant area.