"Boo" Diddly was the name given to these ghosts in the instruction manual for Super Mario Bros. 3. Should the quotation marks be around "Boo," or rather, "Diddly?" He and Morton Koopa Jr. seem to have Americanized joke names given to them during localization. I enjoy going back and rereading the descriptions of things in the instruction manuals for NES games, mainly because they tend to butcher whatever carefully created backstory the Japanese original may have had. The American manuals tend to just take a bunch of the cool cartoon artwork and just jumble it around.
Of course, Boos show up just about everywhere afterwards, so you've surely seen them whilst playing Mario Kart, or pretty much anything Mario has his name in. Boy, there are a lot of Mario games. But since he shows up in a misty spooky mansion most of the time, he seems about right for a Halloween post. That, and I almost forgot to post something in the first place. Whew, I'm done, with only a few hours to spare....