Do you think Orys Baratheon adopting the exact sigil and words of House Durrandon was a bit of lazy world building? It just seems that even if he had no sigil of his own he would want some distinction from the rule of Agrilac the Arrogant. Maybe at least say the stag wasn’t a “crowned stag” before the Conquest.

Not really, I thought it made a lot of sense. He’s a bastard in a society that really doesn’t look kindly to bastards, and he’s just made an unimaginable leap in status to just below that of King and Queens. Not to mention, he’s ruling over a kingdom that has, for 8,000 years, known no prince but a Durrandon to stretch a phrase. 

So borrowing the sigil and words of House Durrandon is a good way to signal that there’s going to be continuity rather than constant upheaval.