The Ingerman Helmet is an object that is often worn by Fishlegs Ingerman.
In "Loyal Order of Ingerman", the Ingerman Helmet was revealed to be a traditional helmet of the Ingermans, who are Dragon Hunters. They are the ancestors of Fishlegs and this is why he wears that helmet. When Fishlegs go to the Dramillion Island, he saw Dragon Hunters and the statue of Ingar Ingerman, who also has the helmet. The Dragon Hunters switched their usual helmets to Ingerman helmets in order to respect Ingar Ingerman. When Fishlegs meets the first Dramillion he encountered, the dragon was scared, because Fishlegs wore a traditional Ingerman Helmet, then he removed it and that dragon is no longer scared. Fishlegs and Astrid freed all Dramillions and destroyed Ingar Ingerman's statue, because he does not want to be like his ancestors.
The helmet appeared to be a Viking helmet with dragon wings instead of horns. It is also made of metal and small compared to most Viking helmets.