I'm thinking of a being with the head of a dog and the body of a fish. I'm not getting mixed up with Oannes either. I'm really quite stumped and can't seem to find anything on Google.. it does seem to have an old Babylonian edge to it though.
Found hereSkylla, who had a dog head and eat the sailors.
Her name comes from skylax ("young dog").
Greeks called the shark "skylion".
Scylla has the face and torso of a woman, but from her flanks grow six dog heads and her body below that sprouts twelve canine legs and a fish's tail.