From the Super Mario Wiki
A Mr. I (originally parsed Mr. I.) is an enemy species in the Mario series. Its name is self-descriptive; they seem to be nothing more than a giant eyeball. Additionally, Mr. I is usually completely immobile, as it can only rotate around in place, though in Super Paper Mario, they gained very slight movement.
Super Mario 64/Super Mario 64 DS
It first appeared in Super Mario 64 where the only thing it can do is shoot strange, white bubbles at Mario from its pupil (In Super Mario 64 DS, this was changed to a blue burst). It always keeps its eye on Mario, and can be dizzied into submission if Mario runs in a circle around it either clockwise or counterclockwise, rewarding Mario with a Blue Coin. The much larger version, Big Mr. I, is in the attic of Big Boo's Haunt. The standard Mr. I can be found in Big Boo's Haunt, Lethal Lava Land, and the Hazy Maze Cave.
Mario Party series
Mario Party 2
Mario Party 3
Mario Party 5
A Mr. I later appeared in Mario Party 5 as a weapon in Super Duel Mode. It would shoot a blue laser to stun enemies.
Mario Party 8
In Mario Party 8, Mr. Is and Big Mr. I appear in the minigame Eyebrawl, where instead of running around them like in the Eye Sore minigame, the player must draw circles around them with the Wii Remote. In this game, Mr. I took on a more oblong shape. Whoever gets Mr. I dizzy first wins the minigame. The loser is haunted by Mr. I.
Mario Party Advance
In Mario Party Advance, a notable Mr. I (simply named Mr. I) is a resident of the Horror Area who is in love with Princess Peach. If the player meets Mr. I while playing as Peach he will give her the Hope Chest Gaddget.
Super Paper Mario
In Super Paper Mario, Mr. I, identified as Mister I, appears in Merlee's Mansion and Castle Bleck; this Mr. I is immune to conventional attacks and can only be defeated by running around it repeatedly while using Mario's Flip technique. Alternatively, the player may use Fleep's ability on it. Like in previous appearances, Mr. I would attack the player by shooting them with a laser beam. This is the only game to feature Mr. I in 2-D. The game also introduces a sub-species of Mr. I called Red I.
Names in other languages