Here be dragons!
"Dragon" is here a cognate from the LXX δράκων (drakōn).
In various ANE mythologies, creation results from the slaying of a sea monster (Hebrew: תַּנִּין tannin; Greek: κῆτος / kētos), and this imagery shows up throughout the Old Testament. Such references, including other mentions of sea monster(s) in the LXX, include Genesis 1:21; 3 Maccabees 6:8; Job 3:8; 9:13; 26:5–13; Wisdom of Sirach 43:25; Jonah 2:1–2, 11; Isaiah 27:1; 51:9–10; Daniel 3:79).
I think this contains an intentional affront to Ugaritic (i.e., Canaanite) mythology. In Ugaritic accounts, Baal battles against Yamm (which is a homonym of the Hebrew word for "sea," yam), the sea monster who is also referred to as Tannun and Litanu. In an Ugaritic account, Baal defeated Yamm as well as the sea itself (yam). The overlap here with biblical terminology is apparent.1
This affront is clearer in vv. 13-17 in the Hebrew, which connects the crossing of the sea in the exodus event to creation, alluding to common ANE creation mythology (just as God defeated the chaotic waters in creation, he again subdued chaos and his people crossed the sea on dry land).
An alternative translation of vv. 12-17 from the Hebrew follows:
But God has been my king from long ago,
working salvation in the middle of the earth.
You parted the sea [יָ֑ם / yam] by your strength;
You broke the heads of the sea monsters [תַ֝נִּינִ֗ים / tanninim] in the waters.
You crushed the heads of Leviathan [לִוְיָתָ֑ן];
you gave him as food to the desert-dwelling animals.
You split open spring and wadi.
You dried up ever-flowing rivers.
Yours is the day, yours is the night also.
You established light and the sun.
You defined all the boundaries of the earth;
Summer and winter—you formed them.
The Psalmist connects the parting of the sea to God's creation of the cosmos, showing how intertwined such ideas were in ANE thought.
- 1Cf. James Bennett Pritchard, ed., The Ancient Near Eastern Texts Relating to the Old Testament, 3rd ed. with Supplement (Princeton: Princeton University Press, 1969), 129–131.