Boo

A Boo is one of King Boom Boo's minions in Sonic Adventure 2 and in "Sonic's Scream Test" of Sonic X. They are red-eyed ghosts with sharp teeth.

Sonic Adventure 2
In Sonic Adventure 2, Boos appear in Pumpkin Hill, Aquatic Mine, Egg Quarters, Pyramid Cave, Death Chamber, and the boss level for King Boom Boo. Their only ability is to choke the player, or chase them down and touch them, losing all their rings. Lots of times, Boos will hide in gravestones, pumpkins, and even just the ground and will randomly pop up and act as if it was attacking the player, but within a short period of time, the Boo will disappear. This was mostly likely intended to scare the player.

Boos also appear either much smaller or much bigger, with cuter eyes and a patched up mouth. But this should not deceive the player, because the small ones will randomly surprise the player with a quick attack and then disappear. With the bigger ones, if the Boo is defeated, 3 more normal ghosts come out, and these 3 must be defeated.

"Sonic's Scream Test"
In "Sonic's Scream Test", the ghosts make many laughing noises. Also, instead of choking others, they possess the host (in this case Amy Rose was the victim). The big ghost with the "sincere-eyes" returns, and is smashed by Amy Rose, once Amy comes to her senses and notices she had been possessed.