Legio XX Valeria Victrix by Steve Malone is licensed under a
Creative Commons Attribution-Non-Commercial-No Derivative Works 2.0 UK: England & Wales License.
Prior to the reforms of Marius in 104BC the Roman legion had a variety of standards, the eagle, the wolf, the minotaur, the horse and the boar which had been carried in front of different elements of the legion (though we do not know which). The eagle had always been the most important, but it was Marius who gave pre-eminence to the eagle and abolished the others. These standards were 'animal totems reflecting the religious beliefs of an agricultural society' (Pliny Natural History 10.16; Keppie 1984). Boars are important generally in Iron Age iconography, they are 'aggressive, indomitable and awsome creatures at bay, strong, fearless and destructive' (Green 1992) and as such their adoption in a military context is unsurprising.
Most of the known legionary emblems are zodiacal in origin, reflecting either a Caesarian origin (the bull - legions III Gallica, IIII Macedonica, VII, VIII Augusta, X Gemina) or a foundation or re-formation under Augustus (capricorn - legions II Augusta, IIII Macedonica, IIII Scythica, XIV Gemina, XXI Rapax). Legions XIII, and perhaps XVI, have a lion emblem. Other emblems seem to reflect service at Actium, or in other sea-battles of the civil war; X Fretensis has a dolphin and galley among others; XI and XXX have Neptune. The pegasus appears for II Augusta and III Augusta. V Alaudae boasts an elephant (from a historical episode). VI Ferrata, the wolf and twins (and perhaps a bull also). Legio XII Fulminata has a thunderbolt, as you would expect from the name.
According to Domaszewski (1885) the boar signifies the god Quirinus, thought to be originally of Sabine origin, a god with martial qualities seen as the personification of the deified Romulus. It was used as an insignia by several legions. Leg I Italica, leg II Adiutrix and leg X Fretensis all made use of it as well as leg XX Valeria Victrix.
Reinach, A.J., 1910. ‘Signa Militaria’ in Daremberg-Saglio, Dictionnaire des Antiquités iv.2, 1307-25
Ross, A., 1967. Pagan Celtic Britain ‘The Boar’ 308-321
Toynbee, J.M.C., 1973. Animals in Roman Life and Art. ‘Boars and Pigs’ 131-136
Green, M.J., 1992. Animals in Celtic Life and Myth.