In Finnish mythology, what demon is a monstrous sea creature?

In Finnish mythology, the demon-like monstrous sea creature is Iku-Turso.

Iku-Turso is a mythological giant or sea monster associated with Finnish and wider Finnic tradition. In the Kalevala, Finland’s national epic, he rises from the sea and is described as a hostile ocean creature. In one episode, Louhi, the Lady of the North, summons him to stop the heroes who have stolen the magical Sampo. Väinämöinen captures the monster, compels him to promise not to return, and sends him back beneath the waves.

The name has several forms, including Iku-Tursas, Meritursas, Tursas, and Turisas. Their meanings and identities are not always consistent: scholars have debated whether the runic songs describe one being or several related giants, sea creatures, and war figures.

Iku-Turso is therefore not the same as Louhi, Hiisi, or Lempo. Those figures occupy different roles in Finnish mythology, while Iku-Turso is chiefly remembered for his gigantic and dangerous marine form.