The chimaera was a three-headed monstrous creature used as a symbol of various individuals and organizations during the waning years of the Galactic Republic and the subsequent reigns of the Galactic Empire and the New Republic.
Originally the symbol of the Marocsaa subclan of the Paccosh, it came to become the personal insignia of the Chiss Grand Admiral Mitth'raw'nuruodo. As such, it came to be associated with Imperial personnel under Thrawn's command and even the namesake and ventral decal of Thrawn's flagship Star Destroyer the Chimaera.
Mitth'raw'nuruodo, better known as "Thrawn," originally came across the chimaera as a Senior Captain of the Chiss Ascendancy around 19 BBY. At the time, most of the galaxy outside the Unknown Regions was ruled by either the Galactic Republic or the Confederacy of Independent Systems; two parties locked in the pan-galactic Clone Wars.
While investigating an attack on a refugee ship at the edge of Chiss space, Thrawn journeyed to the Rapacc system, from where the dead refugees came. There, Thrawn encountered a Paccosh named Uingali foar Marocsaa and his translator, who warned the Chiss that the Paccosh feared extinction at the hands of their conquerers, the Nikardun Destiny. Fearing the loss of his people's memory in the knowledge of history, Uingali entrusted Thrawn with a treasured ring of his clan depicting a chimaera, so that the Marocsaa subclan would be remembered if the worst should happen.
The next year, Thrawn returned to Rapacc, finding its people freed of their Nikardun oppressors. Thrawn met Uingali, by then an official within the Paccian Governance, and saw that his old friend now commanded a captured Nikardun blockade frigate bearing a chimaera on its underside. Thrawn returned the Marocsaa subclan ring to Uingali, but the Paccosh would eventually send it back to Thrawn as a gift in honor of the aid he had given their people which Thrawn accepted out of respect and was one of the few personal items he took with him upon his exile, later coming to adopt the chimaera symbol as his personal insignia.
Senior Captain Thrawn eventually left the Unknown Regions for "Lesser Space" to investigate the potential of the young Galactic Empire to serve as an ally against, and possibly bait for, "evil threats" to his Chiss Ascendancy. Nevertheless, the Chiss retained the chimaera as his personal symbol. As the Chiss rose within the ranks of the Imperial Navy, so too did the symbol of the chimaera.
Upon Thrawn's promotion to Commodore, he was granted command of an Imperial Star Destroyer named the Chimaera, which bore a stylized depiction of a chimaera on its ventral surface. The officers and sentry droids in Thrawn's Seventh Fleet had a stylized depiction of a chimaera on their pauldrons or uniforms.
Morgan Elsbeth was a woman whose people were massacred during the Clone Wars, and who used her fury about that massacre to fuel the construction of the Imperial Starfleet. Elsbeth eventually became a protégé of Thrawn. In around 9 ABY, former Jedi Ahsoka Tano tracked Elsbeth down and waged a one-woman insurgent campaign against Morgan Elsbeth's army to liberate the oppressed people of Corvus and to learn the location of Thrawn. During the Liberation of Calodan, Elsbeth's HK-87 assassin droids bore a depiction of Thrawn's chimaera on the side of their heads. At the time of the liberation of Calodan, the New Republic at least nominally ruled most of the galaxy, having defeated the Empire at the battle of Jakku.
The chimaera symbol was first depicted in Star Wars canon in the 2016 Star Wars Rebels episode Iron Squadron, where it appeared on Thrawn's eponymous flagship. The back-story for Thrawn's introduction to the chimaera as his symbol was first developed in the 2020 novel Thrawn Ascendancy: Chaos Rising by Timothy Zahn, Thrawn's original creator.
Previously, the chimaera had been associated with Grand Admiral Thrawn in the Legends continuity, on Thrawn's Chimaera in that continuity.