One myth that fits into this category is the Babylonian one about Marduk and Tiamat (who later went on to star in the first Final Fantasy game). Tiamat symbolizes primordial chaos, as well as salt water. I suppose the connection between the ocean and chaos is understandable for pre-scuba-and-submarine cultures. There are some hints in the book of Psalms that early Hebrew creation myths also involved Yahweh slaying primordial sea monsters, before the Levites decided that He could call things into existence just by speaking, with no monster-killing necessary. Anyway, Tiamat and her husband Apsu gave birth to a bunch of gods, who were way too noisy. Apsu advised killing them, while Tiamat thought they should just be given broth without bread, whipped soundly, and sent to bed. The god Enki (also known as Ea, because gods are NOTHING if they don't have multiple names), not being too keen on Apsu's plan, killed him, leading Tiamat to desire revenge. Enlil's son Marduk (who had fifty names, as well as four eyes and four ears, at least according to some sources) said he would kill Tiamat if he'd get to be chief of the gods. From what I've found on the Internet, he was originally the patron deity of Babylon, but rose to chief of the gods after that city emerged as the main power in the region. A lot of myth cycles incorporate stories about gods overthrowing other gods, which are quite likely symbolic of changes in religious and political structure. I mean, just look at Cronus castrating his father Uranos (and no, I don't mean LITERALLY, as it would probably be a quite disturbing image), and then being himself conquered by his son Zeus. I've seen it suggested that Tiamat herself might have been a deity worshiped by a more matriarchal society in the distant past. Regardless, the story reports that Marduk killed Tiamat with some help from the winds, and proceeded to forming the world from her corpse. To add a bit of local flavor, the Tigris and Euphrates Rivers were said to come from the tears flowing from her eyes.
Incidentally, when I went to a summer program meant to introduce people to the forthcoming Honors College that I attended, one of the professors drew parallels between the Marduk/Tiamat battle and the movie Aliens, with the alien queen representing Tiamat. They were interesting, as I recall, but I've never actually seen any of those movies, and can't remember much else about the idea. I suppose it's all just part of the archetypal story of the hero defeating a disgusting monster in order to restore (or, in some cases, create) order.
In the Norse creation myth, it's a male monster who's the source of the physical world, and he's made of frost instead of sea water. Chalk it up to differences in the originating lands, I suppose. The giant Ymir came into existence through the interaction of ice and heat, and then he hung around drinking from a celestial cow's udders and sweating out kids. Yes, the Norse giants came out Ymir's arm and leg perspiration, which might explain why they always seem so pissed off. The first god, on the other hand, was licked out of a block of ice by the cow. I give this myth some major points for creativity, but it's kind of gross, isn't it? Anyway, this god's grandsons decided to kill Ymir. But unlike the Babylonian myth, which presented the fight between the gods and the monsters as being one of constant retaliation, I'm not sure there's any actual REASON why Odin and his brothers thought Ymir deserved death. Maybe they were just sick of watching him sweat out offspring. Or it could have been that berserker rage thing. I don't know. Anyway, after killing him, they made the world out of his body, using his skull as the sky and his brains as clouds. Oh, and the maggots that ate his flesh turned into dwarfs. Creation, according to the Vikings, was a rather messy affair.
I'm thinking that I might make this mythology thing a weekly feature. Next time, I'll move on to various explanations of the origins of humanity. If any of you have any other suggestions for future posts, feel free to let me know.