The work contains a number of prophecies attributed to Merlin, the wizard of legend, whose mythical life is often regarded as created by Geoffrey himself although Geoffrey himself claims to have based on older Brittonic traditions some of which may have been oral but now lost. The Prophetiae preceded Geoffrey’s larger Historia Regum Britanniæ of c. 1136, and was mostly incorporated in it, in Book VII; the prophecies, however, were influential and widely circulated in their own right. […]
When Geoffrey’s Historia was largely translated by Wace into the Roman de Brut, he omitted the material on Merlin’s prophecies, though he does profess knowledge of them. […]
This work not only launched Merlin as a character of Arthurian legend: it also created a distinctively English style of political prophecy, called Galfridian, in which animals represent particular political figures. Political prophecy in this style remained popular for at least 400 years. It was subversive, and the figure of the prophetic Merlin was strongly identified with it. […]
In this work Geoffrey drew from the established bardic tradition of prophetic writing attributed to the sage Myrddin, though his knowledge of Myrddin’s story at this stage in his career appears to have been slight. […]
Geoffrey apparently introduced the spelling “Merlin”, derived from the Welsh “Myrddin”. The Welsh scholar Rachel Bromwich observed that this “change from medial dd > l is curious. It was explained by Gaston Paris as caused by the undesirable associations of the French word merde”.
Source: Prophetiae Merlini – Wikipedia