In Rome's Forum of Trajan fragments from architraval friezes with figurative motifs of seven different types have been discovered over the centuries: eagle-headed griffins and candelabra; eagle-headed griffins and tripods; eagle-headed griffins and cupids; lionheaded griffins, cupids and vases; cupids in acanthus; Victories killing bulls and dressing candelabra; sphinxes. The aim of this research is to investigate if there is a connection between these decorative motifs and the architectural structures where they were exhibited, and if their iconographic choice depends on their deeper significance in the Forum's wider context. Following an introduction to the history of the excavation of the Forum and to its different sections, for each frieze type is proposed a comprehensive analysis of all the fragments with an ascertained provenance from the Forum, which have never previously been studied together. Their location in the different buildings forming the Forum's complex, as identified through information from excavation reports, drawings and the relevant reconstruction of the architectural orders is discussed, as well as their iconography and comparison with other artworks and monuments, which attests that Trajan chose a traditional iconography already used in previous similar examples in contexts comparable to the various structures of Trajan's Forum. Against this background, we argue that the reliefs were part of a wider figurative programme, and Trajan expressed the messages he wanted to transmit through the traditional language of his predecessors. Griffins, cupids, Victories and sphinxes are polysemous mythological figures, attributes of different divinities: their symbolism related to diverse gods depending on their various aspects can be interpreted according to the function of the buildings forming the Forum when it is known, or can help to investigate the use of these structures when it is not known.