Things Fall Apart; My Memory Cannot Hold

I think it is fair to say that I have a pretty good memory, but when I looked at the title of William Yeats' poem "The Second Coming," nothing came up my mind. So I opened the textbook, revisited the poem, and I knew why: I don't really understand it. I got frustrated until reading what SparkNotes says:

Because of its stunning, violent imagery and terrifying ritualistic language, “The Second Coming” is one of Yeats’s most famous and most anthologized poems; it is also one of the most thematically obscure and difficult to understand. (It is safe to say that very few people who love this poem could paraphrase its meaning to satisfaction.)

Yeats believes that history occur in two-thousand-year cycles. After the period of Christianity which was marked off with the birth of Christ, a new era would be inaugurated by another figure. A "rough beast" (21) with "lion body and the head of a man" (14), perhaps? If his beliefs are true, then the second coming should have happened by now. Who is the beast?

A possible answer below...?
http://www.cartoonink.com/2008/12/02/what-rough-beast/