From Jeffrey Greene's The Golden-Bristled Boar: Last Ferocious Beast of the Forest:
The Romans associated the Gauls with boars. On Trajan's column, a boar is a Gallic emblem in the depiction of Trajan's triumphs in Rome's northern provinces. Gallic warriors invoked the boar's spirit to lead them aggressively into battle and inspire cunning maneuvers on defense. A boar's hide was thought to impart healing powers to the wounded.
But ironically, as I know from living in Chester (Roman Deva), home to the Twentieth Legion:
Romans shared the Gallic enthusiasm for the boar emblem. The boar served as an insignia for Octavian's Twentieth Legion campaigns in what is now Spain, Germany, and Britain, as well as for his Tenth Legion during the Balkans campaign. Nero's First Legion, in its defense of the Danube, also fought under the boar insignia.
Here's a Roman antefix (roofing end tile) found close to Chester, with the Twentieth Legion's (Legio XX) boar emblem: