Dragon God from the Sumerian creation myth.  God of the fresh water.  He and his mate, Ti'amat (Dragon Goddess of fresh water) gave birth to the gods.  Their children were noisy and chaotic, and Apsu plotted with his vizier to kill the children.  One of the children,  Enki, God of Magic, cast a spell on Apsu and killed him.  Ti'amat was enraged, went to battle against her own children.  Eventually, she was drawn in to single combat with Marduk, child of Enki and his wife, Damkina.  Marduk slays Ti'amat, and creates the firmament from her body.

Also, the name of my (correction, our) Black Arowana.

It may also be worth noting that "apsu" or "absu" is the Babylonian word for well (as in the above mentioned fresh water). Wells and springs are often symbolic of deep and hidden knowledge, such as that which might be found with those that are older than the gods (see Prometheus and the other Greek Titans).

The soft link below to the word abyss makes me think it might be quite possible that this, too, is etymologically related. Just a speculation.