Are there any limitations to 'pen names'?

A lot of novelists have pen names (Lemony Snicket, J.K Rowling, ect.) I was wondering, are there any limitations to pen names? Like, do pen names HAVE to be ordinary, average, human names, or can a pen name be like a thing, like ''Treasure'', or something more imaginative and exotic, like ''Salatrinaria''? And can a pen name be just one word instead of two?