Junior Shrooboid

The Junior Shrooboid is a green Shroob who appeared in Mario & Luigi: Partners in Time. During the events of the game, this strange creature used Professor E. Gadd's time machine to travel from the past to the present. Eventually, Junior Shrooboid winds up in Princess Peach's throne room. At first, he does not move. As soon as he does, immediately Mario confronts the alien (while Luigi runs away scared). In battle, the Junior Shrooboid has 13 HP and attacks by spinning into an opponent. Unfortunately for the intergalactic fungus, Mario could easily Jump over this attack and stomp on the Junior Shrooboid. After the battle, Professor E. Gadd places the Junior Shrooboid in a fluid tube, keeping the alien alive for study. E. Gadd discovers that the Junior Shrooboid reacts strongly to the presence of Cobalt Star shards. The professor hypothesizes that the Cobalt Star could be used to defeat the Shroobs, and commands Mario and Luigi to recover the remaining star shards. Unfortunately for the heroes, the Junior Shrooboid reacted so strongly not because the Cobalt Star could defeat the Shroobs, but because the Junior Shrooboid was aware that Elder Princess Shroob was imprisoned inside the artifact itself. It is unknown what happened to Junior Shrooboid at the end of the game, and it is possible that Professor E. Gadd is still studying it.

The battle with the Junior Shrooboid was merely a tutorial battle for Mario to develop his fighting techniques, similar to Baby Bowser's battle. It is impossible to lose against Junior Shrooboid.

Trivia
In Mario & Luigi: Bowser's Inside Story, while Mario and Luigi battle the Shroobs, some Junior Shrooboids can be seen frozen in blocks of ice.