A little while ago, I put up a couple of posts on the transmission of classical texts and the Carolingian Renaissance. Recently I needed to look at Horace's second Epode and had recourse to David Mankin's commentary in the 'Cambridge Greek and Latin Classics' series. In section 7 of the introduction, he has the following to say about Horatian manuscripts:
It appears that after their publication in the late first century BC Horace's works went in and out of fashion but never ceased to be read in parts, if not the whole, of the Roman world. Nevertheless, the earlies manuscripts (MSS) containing them date only to the ninth century AD, when there may have been a kind of 'Horatian revival' at the court of Charlemagne. These MSS seem to be the ancestors of the countless others from later centuries, and are the only ones that modern scholars draw on for composition of the text.