Karl Jenkins was a Hogwarts student in the same year as Albus Potter and Scorpius Malfoy, to each of whom he referred by an unkind nickname (CC1.4).
Karl is one of the students who expressed surprise when Albus Potter is sorted into Slytherin. He also seemed to believe the rumor the Scorpius was Voldemort's son (CC1.4).
Karl has at least one magical parent (CC3.2).
gives very hard, painful high-fives (CC3.2)
Given that he is the first person we hear call Scorpius "Scorpion King" and that in the main timeline he referred to Scorpius only as "Voldemort's child" and nicknames Albus "the Slytherin squib," it may very well be Karl who originally comes up with the nickname "Scorpion King."