Loki is the Norse god of thievery and mischief. He is known for causing chaos. Although rarely mentioned in the books, Hiccup prayed to Loki when trying to steal a baby dragon. Also 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".
- "Oh, Loki, please let it be so! We will welcome him with open arms. [ominously] Teach him the ways of the truly disturbed!"
- "Thank Loki, you guys are not gonna believe what just happened to me, it was-"
- "Tuffnut: Wait. I think I just had another idea. Whoa! Do you know how many Loki-ish thoughts are running through my mind right now?
Astrid: Tuffnut. This is not the time for pranks, or Loki-ing."
It is shown in Shock and Awe 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.
- "Tuffnut: On Loki Day?
Ruffnut: The day of our favorite god's birth?
Tuffnut: One of the greatest days of pranking ever conceived by any man, woman, child, animal, vegetable, mineral or rock?"
- 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".