It was indeed Grimbeard the Ghastly's famous shield, taken in battle by the Bog-Burglars many years before and held by the Tribe as a trophy ever since.
— Book 3
In How to Speak Dragonese, the Romans try to pit the Hooligans against the Bog-Burglars by kidnapping the heirs and blaming the other tribe. Stoick sails to Fort Sinister to find his son. Bertha, however, followed Stoick with the Bog-Burglar navy, thinking Stoick is running away from a blood feud. While the two are arguing, a Roman Observation Balloon crashes aboard The Blue Whale and out crawl Hiccup, Camicazi, and Fishlegs. With relations restored, Bertha gives the Hooligans the shield from Grimbeard the Ghastly. Snotlout grabs it and holds it up triumphantly, even though he had nothing to do with the heirs' return. Camicazi instructs Fishlegs to push him, which he does. Top-heavy from holding the shield, Snotlout falls into the sea and gets laughed at by everyone.
Grimbeard's shield is large and round, as well as ornately decorated.
Perfectly round, in the center was a skull crowned by seaweed, around which waves and dragons chased each other in an endless circle.