The African diaspora traditions of extemporaneous declaiming, dozens, and rapping predate the emergence of US rap. And my favorite manifestation of it is Trinidadian calypso from about 1930 - 1960.
This compilation of The Mighty Sparrow & Lord Melody's answer songs to each other provides a great example of the braggadocio and scorn-heaping experienced calypsonians are capable of.