I've never heard or seen it, and Google turns up only your post, so I would say that this phrase doesn't exist as an expression in English. Please give us the full sentence in which you heard it or read it and tell us who said it, and in what situation. Without that, we can't even guess at what it might mean.Is there really any idiom called 'to rag on the beast'? IF so, what does it mean?
Rag, meaning "scold," has been around a long time - since at least 1739, according to the Online Etymology Dictionary, and it is sometimes used with "on." It doesn't appear to be etymologically related to rag, meaning "scrap of cloth," nor to the phrase "to be on the rag," which dates from 1948 and means "to menstruate" or "to be really cranky and bitchy." The latter meaning can apply to either men or women, by the way, though no doubt the supposed crankiness women are supposed to feel while they are menstruating is the origin of that expression.
But I've never heard rag on the beast, either. Some context would definitely help. The part about "the beast" is particularly cryptic.