Loki

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
Three years after the war with the Berserkers, the Thorston twins, Ruffnut and Tuffnut, worshiped Loki by playing pranks and causing mischief on Berk.

Shock and Awe
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.

Trivia

 * A type of tree that grows only on Outcast Island is called the Loki Tree.