Yoob

Yoob is a giant, Shroob-created Yoshi that mysteriously appears during the events of Mario & Luigi: Partners in Time. The creature known as Yoob originally starts out as an abnormally large, pink-and-green Yoshi Egg. One day, this egg arrives on Yoshi's Island (presumably placed on the island by the Shroobs). Although no one knows where it comes from, the egg instantly becomes a tourist attraction. For five Coins, tourists can actually witness the enormous egg close-up. However, the egg soon hatches, and Yoob is released. Since Yoob is apparently programmed by the Shroobs to eat Yoshis, it starts terrorizing the island, eating every Yoshi in sight.

When the four Mario brothers (Mario, Luigi, Baby Mario and Baby Luigi) first arrive on Yoshi's Island, the place is deserted. Most of the Yoshis have already been eaten. The few survivors are hiding in a small tree hut. Soon, Yoob discovers the the Yoshi hideout. The Yoshis attempt to flee, but Yoob eats all of them, even Yoshi. The Mario brothers survive the attack. With no more Yoshis left in the area, Yoob leaves to search for more food.

Later, while climbing a cliff on during its food search, Yoob is blasted with an energy beam from a Shroob UFO. The beam causes Yoob to grow to gigantic proportions. Yoob, who is originally about ten times bigger than Mario, is now over a hundred times bigger than him. In fact, Mario is now smaller than one of Yoob's eyes. It eats the Mario Bros and Baby Bowser as well.

Once inside Yoob's Belly, the brothers discover that Yoob's stomach is not a normal digestive system. In fact, part of its stomach has been transformed into a bio-mechanical factory by the Shroobs themselves. The purpose of the factory is most likely to create even more Yoobs, which can then be released against other Yoshi-inhabited Mushroom Kingdom. After Yoob eats a Yoshi, the RC Shroobers handle the Yoshi. Then, the Yoshi is 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 are fragile. Soon, the four Mario brothers put an end to the operation. To do so, the brothers aid 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 manages 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 is created by the Shroobs to protect the eggs from being stolen. After a long battle, the brothers defeat Sunnycide, and, with the help of a Toad named Toadbert, free 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 needs the energy provided by the Yoob eggs to remain operational, falls asleep. All the captured Yoshis are 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.

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 will not happen if the player defeats Sunnycide.
 * The word "Yoob" is a combination of Yoshi and Shroob.
 * Kylie Koopa christened the giant Shroob Yoshi "Yoob". It is unknown if the Shroobs themselves gave it a different name upon creation.
 * 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 is really him, though.