Boo! is a cancelled platform game that was in development by Micropose for the Super Nintendo, Genesis / Mega Drive and Amiga in 1994. As we can read on the website of Boo!’s producer Stuart Whyte:
Designed by Richard Lemarchand, Boo! was a side on platformer inspired by Sonic and Mario, we wanted to create a universe with the central character being a cool ghost called Boo who’s default method of attack was to shout “Boo!” at an enemy to scare him away. Working with Keith Scoble (from Cosgrove Hall and creator of Jamie and the Magic Torch and Dangermouse) we built a great game… but unfortunately, due to rocky financial times at Microprose, we never had the money to release – so the game has been lost in time
You can find more info and screens in this article by SNES Central!