I'm sorry but I don't know any medieval propaganda off the top of my head, but an excellent example of Roman propaganda is Cicero's Phillipics, especially his 2nd phillipic. These were speeches (though the 2nd was never actually read out and supposedly handed out as a pamphlet) designed as anti-Mark Antony Propaganda, and basically talk about how awful he is. They're quite funny so can be quite an enjoyable read.

Another possible piece of Roman propaganda is the common belief that Carthaginians practiced child sacrefice. It is much debated whether this is true or simply a lie spread by the Romans to justify their destruction of Carthage...

There are plenty more examples in the ancient world - Aeschylus' play the Persians contains lots of anti-Persian propaganda.

I hope this helped, sorry i couldn't give you any for the medieval period though I'm sure there was plenty anti-french Propaganda in the 100 years war by the English. The main difference between modern propaganda and pre-modern is that pre-modern tends not to be commissioned by the government, and instead spread as rumours or is created by an individual, but no doubt there are exceptions to this.

Hope this helped at all

Augustus Caesar, first emperor of Rome Augustus Caesar supposedly wrote and circulated this little gem (Martial 11.20): "Because Antony fucks Glaphyra, Fulvia has arranged this punishment for me: that I fuck her too. That I fuck Fulvia? What if Manius begged me to bugger him? Would I? I don't think so, if I were sane "Either fuck or fight", she says. Doesn't she know my prick is dearer to me than life itself? Let the trumpets blare!"

So as far as propaganda as slander goes, it has a long and illustrious history.