Lava Louts are the Hairy Hooligans' great enemies. They are sometimes called 'snakes in helmets'.
When Humungously Hotshot the Hero first arrives in How to Twist a Dragon's Tale, the Hooligans believe he is a Lava Lout, as he has protection from the flames of the fire he came from. The Lava Louts had already fled the Island, so they are never shown.
The Lava-Louts suits are made from Dragon-skin allowing them to be safe from fire (A great necessity since they live on a volcanic island) and completely cover the body. Even a lava-lout cloak gives them protection against flames, with it being said it was as cold as the ocean and smell comfortingly of fish (How to twist a dragon tale page:54, page 65).
It was said that they were cruel slavers. This fact was reinforced when Hiccup, Toothless, Fishlegs and Camicazi went to the Lava-Lout Village, there were cages everywhere, manacles, chains, whips, weapons of all description. Huts with barred windows with beds of stone or iron (Pages: 149-150). They Humongously Hotshot forge swords with them and using both humans and dragon to dig in the Lava-Lout Goldmines (Which was hinted as to what Hiccup WindWalker escaped from (Page 18.)
The Lava Louts are briefly mentioned in How To Steal A Dragon's Sword, as Stoick remembers the reason why he walks with a limp, is because of a fight with the Lava Lout.
They are never seen in any of the books.
Their is a island called Lava Lout island in the game Wild Skies