According to Psalm 104:26, God formed which creature to frolic in the sea?

Answer: Leviathan

Psalm 104:26: "There the ships pass, and Leviathan, which You formed to frolic there" (KJV: "to play therein").

“There the ships pass, and Leviathan, which You formed to frolic there.”— Psalm 104:26 (BSB)