That's an interesting thought, too; though I always saw the mural more as a reference to the God and Adam from the Sistine Chapel, implying that the creature (dog-alien or not) was either a creation of the Engineers or some kind of subservient species. Though it is entirely possible that the Engineer is trying to restrain the creature.
But looking at the "Art of Prometheus" book, the Engineer does have a wavy line that could easily be a scar or wound on his abdomen, where the liver would be. A mystery wrapped in an enigma.
It is interesting to speculate as to whether the Engineers got the xenos from the dog-aliens, or vice versa!