Loki is the Norse god of thievery and mischief. Although rarely mentioned in the books, Hiccup prayed to Loki when trying to steal a baby dragon. Also in between Dragons: Defenders of Berk and Dragons: Race to the Edge, Season 1, the twins dedicated their lives to Loki by pranking everyone on Berk. ("Dragon Eye of the Beholder, Part 1")
Loki is the twins' favorite god. They're quite often praying to, taking to, and thanking him for anything they deem as "fun".
It is shown that there is a holiday called Loki Day, which celebrates Loki's birthday by pulling pranks on others. When the prank is accomplished, the instigators shout "Loki'd!" at those who fell for their pranks.
- A type of tree that grows only on Outcast Island is called the Loki Tree. ("We Are Family, Part 2")
- There's also a Loki Island, where in Dawn of the Dragon Racers, Stoick learns of a special wood that grows there that's great for building Regatta ships out of.
- Loki is also mentioned in the How to Train Your Dragon Book series. Before entering into the Dragon Nursery, Hiccup "muttered a prayer to Loki, the patron saint of sneaky exploits".