Iolo is now infamous for forging poems in the name of Dafydd ap Gwilym and others. He also forged medieval chronicles and numerous triads, as well as counterfeit traditions - great and small - concerning the history of Glamorgan. Bardism and its basic elements, the Glamorgan Classification and the bardic alphabet Coelbren y Beirdd were all figments of his imagination.
Like other well-known Romantic forgers in Europe, Iolo was driven by patriotic and local impulses. He embroidered the fabric of the genuine bardic tradition in order to defend the reputation of the language, literature and culture of Wales and Glamorgan.
Iolo was also driven by more personal motives. He was a sensitive creature who could not accept criticism. By attributing his work to other poets and authors he could distance himself emotionally from his own work and, therefore, minimize the pain if that work was judged harshly by others.
His addiction to laudanum may explain Iolo's inability to differentiate between fact and fiction in his bardic vision. Although men such as John Walters, David Thomas (Dafydd Ddu Eryri), Edward 'Celtic' Davies, and Walter Davies (Gwallter Mechain) suspected the truth of some of Iolo's claims, he was not 'outed' as a literary forger in his lifetime. The extent of his interference with genuine bardic tradition and its texts was posthumously revealed in the early twentieth century.
For further information, see the section on Iolo's legacy and Mary-Ann Constantine, The Truth Against the World (Cardiff, 2007).