Huh, you know I just assumed it had belonged to his bio-father the Cretan Bull. It was, eventually, sacrificed to Athena at Athens by having its throat slit, so it does tie in with the themes of Asterion being used by the gods for political ends, that which should have been Poseidon's going to Athena and her Athens. But then again, why would the cow effigy trample it if it wasn't Asterion's? Though then again again, the valley seems hostile to anyone who isn't the master so that could simply be another misdirection.