Dagur the Deranged wears a specific helmet during the TV series, Dragons: Riders of Berk and Dragons: Defenders of Berk. At this point, he is Chief of the Berserker Tribe. After his down fall, he is not seen wearing a helmet at all.
Dagur's Helmet has no special purpose other than as decoration and head protection. It has no special features or hidden abilities.
Dagur, and of course his helmet, make their debut appearance in the episode, "Twinsanity".
Dagur's helmet usually doesn't move much, but in the episode "Smoke Gets in Your Eyes", a Smothering Smokebreath dragon swipes it completely off his head, along with anything else metal that he's wearing. It's the first time we see him without his helmet.
While Dagur the Deranged does not appear in the game, School of Dragons, his helmet does as a wearable accessory for players to customize their avatars with. It comes in a standard form like seen in the TV series, and a Gronckle Iron variety.