A Blindfold Boo
Blindfold Boos are boos that cannot see, but can hear Yoshis by the Yoshis making noise. They're invincible and only appear in one level of Yoshi's Story. There's a Boo called Boo Man Bluff that appears in Super Mario World 2: Yoshi's Island that acts the same as Blindfold Boos. Blindfold Boos also appear in Super Princess Peach, but their design is slightly different from the Yoshi's Story version.
Blindfold Boos only appear four of them on Ghost Castle 6-3, with light blue blindfolds and big pink ears on his face. As they cannot see where Yoshi is located, they trust in their auditive senses for attack when Yoshi is making noises (such as run, jump, walking, collecting coins or making ground pound). These boos are invincible and to pass them, Yoshi has to walk softly. When Yoshi makes noises in the vicinity of Blindfold Boo, he makes movements to find Yoshi and when he manages to perceive the sound, Blindfold Boo with his laugh starts to attack Yoshi and in case Yoshi runs, Blindfold Boo chases him til Yoshi to stay still or get to a high part where Blindfold Boo can't reach him.
Just like other enemies, an attack by Blindfold Boo, will cause Yoshi to lose three petals on the Smile Meter. If Yoshi touches him somehow, Yoshi will lose three petals on his Smile Meter. The Blindfold Boos can defeat Yoshis, if they beat any Yoshi til four times in a row, depending the Smile Meter, in this case, the Yoshi defeated by the Blindfold Boo will be sent to Baby Bowser's castle and will consequently lose a life with the Smile Meter's petals depleted. If this is the last Yoshi, the player gets a Game Over.