Thor and the Giantess (Geirðröðr)
Thor, disguised as a servant, went to the home of the giantess Geirðröðr to win her as a wife. He lifted the cat (which was actually the world-serpent Jörmungandr in disguise) and drank from a horn that was bottomless (the ocean). He was matched by the giant Utgarda-Pöki, who turned out to be the giant who had been deceiving him with illusions [S2]S2 · Snorri Sturluson, Prose Eddac. 1220–1235textOpen source ↗.
The beliefs are pre-Christian (attested archaeologically from the Viking Age); the written sources are Christian-era (1220–1300) and post-hoc. The gap between belief and text is part of what this section presents.
The 'cat is Jörmungandr' detail is one interpretation; the sources are not entirely explicit [S2]S2 · Snorri Sturluson, Prose Eddac. 1220–1235textOpen source ↗.