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. The remaining Yoshis flee to safety.
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 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.
The Mario Bros. see Kamek, who says Prince Bowser is on top of the cliff and flees, taking some Yoshi cookies with him. The bros. chase him and eventually find Prince Bowser. The babies eat his cookies, so he eats the shards he stole from the bros. and runs away. The bros. chase him and start trying to get the shards out of him when two Shroob UFOs appear. They fly off to Yoob, who is climbing the outside of the cliff, and zap him with a ray, which makes him huge, then fly away. He then proceeds to eat Baby Bowser and the bros.
Once inside Yoob's Belly, the brothers discover that Yoob does not have a normal digestive system. In fact, part of Yoob's Belly is an egg factory, created by the Shroobs. The factory has Yoshis that get trapped in Yoob-like eggs on a conveyor belt which takes them to Sunnycide. After Yoob eats a Yoshi, the RC Shroobers in his belly 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 monsters, destroying the kingdom. However, the eggs are fragile and Mario and Luigi can destroy them with their attacks and free the Yoshis, as shown in the boss battle.
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 encounter the guardian of the Yoob eggs, Sunnycide. Sunnycide guarded the exit, preventing the bros., Toadbert and the Yoshis from escaping, forcing the bros. to fight. 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 Mario Bros. and the Baby Mario Bros., 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 monsters.
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.
Yoob looks identical to a pink Yoshi, except it is much larger and fatter (even bigger when the Shroobs zap it), has black spines, a black shell, blue shoes, green underbelly, black irises, red pupils, and pincers.
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