A great sea creature or dragon — used of literal large water creatures and of mythological sea monsters representing chaos. Egypt is called a tannin (Ezek 29:3). God's power over the tannin shows cosmic sovereignty.
Ancient Near Eastern mythology featured sea monsters as chaos-beings. Israel's God commands these creatures (Job 7:12; Ps 148:7) and will ultimately slay Leviathan (Isa 27:1). Creation tames the chaos.
Revelation 12-13 uses dragon imagery drawn from this tradition — the dragon is Satan, the chaos-power opposing God. Genesis 1:21 declares God created the great sea creatures — he is not in contest with them; they serve him.