This would be an ideal application for an old-school manual (hand hammer driven) impact driver. Most I have ever seen have a 3/8" square "output". Does that happen to be size of your plug?
JD 550g Dozer, Ford 655c, 690e JD excavator, 710g JD Loader backhoe, and more coming and going
File or grind it square again on 2 sides and make it fit a open end wrench. Then a pipe wrench if no go. Then if not out, drill it out and use a tapered easy out.