Yoob

The Shroobs developed an efficient method to prevent the Yoshis from helping the Mushroom Kingdom. Yoob is a giant, Shroob-created Yoshi that mysteriously appeared during the events of Mario & Luigi: Partners in Time. The creature known as Yoob originally started out as an abnormally large, pink-and-green Yoshi Egg. One day, this egg mysteriously arrived on Yoshi's Island (presumably placed on the island by the Shroobs). Although no one knew where it came from, the mysterious egg instantly became a tourist attraction. For a mere five Coins, tourists could actually witness the enormous egg close-up. However, the egg soon hatched, and the monstrous Yoob was released. Since Yoob was apparently programmed by the Shroobs to eat Yoshis, the monster started terrorizing the island, eating every Yoshi in sight.

When the four Mario brothers (Mario, Luigi, Baby Mario and Baby Luigi) first arrived on Yoshi's Island, the place was deserted. Most of the Yoshis had already been eaten. The few survivors were hiding in a small tree hut. Unfortunately, the Yoshi hideout was discovered by the massive creature. The Yoshis attempted to flee, but ultimately every last one was eaten by the massive monster. Even Yoshi, who bravely attempted to fight the massive monster, was eaten by Yoob. Fortunately, the Mario brothers survived the attack. With no more Yoshis left in the area, Yoob left to search for more food.

Later, while climbing a cliff on during its food search, Yoob was blasted with an energy beam from a Shroob UFO. The beam caused Yoob to grow to gigantic proportions. Yoob, who was originally about ten times bigger than Mario, was now over a hundred times bigger than the heroic plumber. In fact, Mario was now smaller than one of Yoob's eyes. And with an enormous appetite to fit his new body, ate the Mario Bros and Baby Bowser as well.

Once inside Yoob's Belly, the brothers discovered that Yoob's stomach was not a normal digestive system. In fact, part of the monster's stomach had been transformed into a bio-mechanical factory by the Shroobs themselves. The purpose of the factory was most likely to create even more Yoobs, which could then be released against other Yoshi-inhabited Mushroom Kingdom. After Yoob ate a Yoshi, the dinosaur would be brought to the internal factory and tortured by RC Shroobers. Then, the Yoshi would be encased in a pink and green egg, similar to the one Yoob emerged from. This would presumably have turned the Yoshis into more Yoobs, strengthening the Shroob forces. However the eggs were fragile and not difficult to break to free the Yoshis inside.

Thankfully, the four Mario brothers put an end to this vile operation. To do so, the brothers aided a group of Yoshis (led by Yoshi himself) in their efforts to escape Yoob's body. By using Chomp Rocks that Yoob had eaten, the Yoshis managed to push the rolling stones through Yoob's intestines, creating a pathway for the Mario brothers to the heart of the internal factory. In the center of Yoob's stomach, the Mario brothers encountered the guardian of the Yoob eggs, Sunnycide. Sunnycide was created by the Shroobs to protect the eggs from being stolen. After a long battle, the brothers defeated Sunnycide, and, with the help of a Toad named Toadbert, freed the remaining trapped Yoshis. Escaping through Yoob's digestive tract, the Marios, Yoshis, Toadbert (and even Baby Bowser) made it back safely to Yoshi's Island. Yoob, who needed the energy provided by the Yoob eggs to remain operational, fell asleep. Fortunately, all the captured Yoshis were saved before they turned into more Yoobs.

If Mario and Luigi return to Yoshi's Island, they can find Yoob's now deactivated body attached to the side of a cliff. In fact, the Mario brothers can actually reenter Yoob's body to find items, badges, and beans the heroes may have missed during their first time through. Since Yoob's body was now deactivated, the brothers don't need to worry about being digested.

It is unknown if the original Yoob started out as a normal Yoshi that was then encased in a Yoob egg, or if Yoob began as a unique genetic experiment created by the Shroobs.

Trivia

 * Yoob bears a loose resemblance to Barney the Dinosaur, the host of Barney and Friends, an educational children's program. As a matter of fact, a Nintendo Power issue predating the release of Mario and Luigi: Partners in Time mentioned the similarities in a pre-release screenshot of the game.
 * When the Babies dig underground or attack with the hammer inside of Yoob's Belly, the player can see the Yoob suffering in pain on the top screen. This won't happen if the player defeated Sunnycide.
 * The word "Yoob" is a portmanteau of Yoshi and Shroob.
 * Kylie Koopa baptized Yoob with that name. Maybe Shroobs called him otherwise
 * In Mario & Luigi: Bowser's Inside Story, an enemy named Fawful Guy has an attack where it is eaten and turned into an egg by an off-screen creature. The off-screen creature's roar is uncannily similar to Yoob's and the egg's color is very similar to the eggs Yoob lays. It is unknown if it's really him, though.