Ah, I've always looked forward to this opportunity...
Outside of the musician's lingo, when you think of the word(s) beast/monster, you think of a creature whose physical capabilities exceeds those of a human's. You also think of a creature whose appearance is grotesque...
So, keeping that in mind, I would consider a musician a 'beast/monster' if his/her abilities
seemed to have reached the apex of human potential. Or, in more simple terms, someone whose skills/creativity are generally considered to be extreme. While whose/what considered a beast/beastly varies from person to person, if you get the feeling that said individual has 'no life' or 'need's a girlfriend/boyfriend', you were probably in the presence of a beast, lol....
Virgil Donati comes to mind. Whether they feel his playing or not, I'd imagine anyone who knows the least bit about the technical side of drumming, would agree that he has attained an unusual level of skill...
I've heard a lot of musicians in my day, but I'd personally classify very few as being "beastly"...
