Codice of Chimeria
Codice of Chimeria was a long-dead barbarian hero from Chimeria. Hrun the Barbarian, who also hailed from Chimeria, claimed that he killed the last dragon 200 years before the events of The Colour of Magic.
This poses a few problems, since swamp dragons are definitely not extinct, as shown in many later books (as Sybil Ramkin breeds and cares for swamp dragons). It is possible that most of the race was wiped out by Codice, and he mistakenly thought that he had in fact killed the last swamp dragon. It's also possible that the swamp dragon actually did become extinct, but due to the actions of the History Monks the race exists in the present day.
An alternate explanation is that Codice slew (what was thought to be) the last noble dragon, a far more heroic deed than killing a pathetic swamp dragon; but the dimwitted Hrun confused the two species.
'Chimeria' is an allusion to Cimmeria, the homeland of R. E. Howard's hero Conan, mixed with 'chimera', a beast from classical mythology whose name today is used to refer to an illusion, a non-existent thing. 'Codice' appears to be derived from codices, the plural of codex, Latin for a bound book and often used to refer to ancient manuscripts, e.g. Codex Sinaiticus.