Top Ten Scariest Mario Enemies

Before anyone adds, yes, Bosses and Mario RPG enemies count
The Top Ten
1 Mad Piano (Super Mario 64)

Very, very scary.

2 Phanto (Super Mario Bros 2 (English Version))
3 Unagi Ell (Super Mario 64
4 Super Dimento (Super Paper Mario)
5 Mimi (Super Paper Mario)
6 King Boo (Luigi's Mansion) In the Super Mario series, King Boo is the leader of all the Boos and a major antagonist. He is an ally of King Bowser. He is depicted as a large white Boo with a crown similar to that of Princess Peach. In the Luigi's Mansion series, King Boo serves as the main antagonist and Luigi's arch-nemesis. He is depicted as gargantuan, even larger than his Super Mario counterpart, with darker eyes, a purple tongue and pupils, and a golden crown with a corrupted power-enhancing jewel. He is considered to be one of the series' most evil villains.
7 Baby Bowser (Yoshi's Island) Baby Bowser, referred to simply as Bowser, is the infant version of Mario's arch-nemesis Bowser, and the main antagonist of the Yoshi's Island series.
8 The Shadow Queen (Paper Mario T.T.Y.D
9 Bouldergeist (Super Mario Galaxy)
10 Fury Bowser (Bowser's Fury)

It gets scary when he jumps in front of you.

The Contenders
11 Kingfin (Super Mario Galaxy)
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