Ug

Ug is the ninth boss encountered in Luigi's Mansion 3, a caveman who possesses a T-Rex fossil and guards one of the Toads' portraits as well as the Basement 2 elevator button, though unlike the others he does not directly possess it with the Toad he is guarding holding it instead.

Appearance
Ug is a muscular yellow ghost bearing an appearance similar to that of a caveman. He has a slightly hunched posture, red eyes and mouth, and black hair.

Luigi finds Toad's portrait and is about to free him. However, the T-Rex fossil comes to life and proceeds to consume the painting before Luigi is able to do so.

After the T-Rex fossil is destroyed, Ug reveals himself while stunned, although he quickly recovers and uses his bone as a club to cause a shockwave.

Battle
The first phase deals with the T-Rex fossil. The player has to launch egg fossils at the T-Rex fossil, stunning it long enough to expose its chest and shoot another egg to crack its vertebrae. After doing this once, only one egg will be available, therefore the T-Rex must chew on Gooigi for its chest to be vulnerable. Eventually, it will become more mobile, with the player now needing to use good timing to launch various fossil parts at the T-Rex to weaken it enough to collapse in a heap.

The second phase deals with Ug himself. The player needs to bait him into using his shockwave attack, which will stun him long enough trying to lift his club back up to allow the player to flash him and then suck him in. Upon being defeated, he will be sucked in, with his throwing the club in the air and humorously have the club land on his still-exposed head before being sucked in, with Luigi releasing the Blue Toad, who is revealed to have the Basement 2 button.

Trivia

 * The first and second phases of Ug's battle is a reference to the 2006 film A Night at the Museum, specifically the part where the protagonists get attacked by a fossilized T-Rex in the museum.