Later folk legends circulating locally in Jämtland claimed the monster was a product of tinkering by two trolls, and that it was a cat-headed creature with a black serpentine body.

[13] The earliest description is of a sea/lake-serpent (sjöorm) inhabiting Storsjön, contained in a manuscript by Morten Pedersen Herdal dated to 1635.

[17][d][e] A version of the legend was also recorded in the group of writings associated with the Rannsakningar efter antikviteter [sv] (‘Researches into Antiquities’),[21][22] specifically in a piece dated 1685 by Anders Plantin (Andreas Plantinus):[f] "It is said that beneath this [rune]stone lies a dreadfully large head of a serpent and that the body stretches over Storsjön to Knytta [sv] village and Hille Sand[g] where the tail is buried.

According to this lore, "A long, long time ago", two trolls named Jata and Kata were on Storsjön's shore, each one brewing his cauldron at the lake for "days, weeks and months", until at last was heard "a groaning sound as if from a sick child" followed by a "loud bang.. as if by a violent thunderclap", and out of one cauldron leaped out a strange creature with a black serpent-like body and a cat-like head, which disappeared into the lake (the trolls and the cauldrons too vanished as well, in a wisp of white smoke, leaving a gaping abyss).

The monster dwelling in the lake became a menace to surrounding areas, and grew so big as to encircle the entire island of Frösön and be able to bite its own tail.

According to one informant, the rune-master was named Gudfast, which thus makes connection with the legendary Austmann Gudfastsson (Austmaðr, Guðfastar son, aforementioned, inscribed on the runestone).

After several sightings, an enterprise of locals was founded to catch the monster in 1894,[39] even obtaining the sponsorship of King Oscar II.