Rashi on Numbers 21:34/רש"י על במדבר כ״א:ל״ד

1אל תירא אתו FEAR HIM NOT — God said this concerning Og but not about Sihon (cf v. 23), because Moses feared to wage battle, perhaps the merit of Abraham with whom Og had been associated would stand up (i.e., be an advocate) for him, as it is said, (Genesis 14:13) “And the one who escaped came [and told it to Abraham]” — this was Og who had escaped from the Rephaim whom Chedoladmar and his allies had smitten in Ashoreth Karnaim (Genesis 14:5), as it is said, (Deuteronomy 3:11). "For only Og king of Bashan was left of the remnant of the Rephaim" (cf. Midrash Tanchuma, Chukat 25, Niddah 61a).