The Chanson de Roland itself was a heavily fictionalized account. What actually happened was that a French scouting party, possibly containing the historic Roland, was bushwhacked and wiped out by Basques at Ronceveaux. Some three centuries later, Turold "set down" ("declinet") the epic we know, whether from an orator or from memory we *don't* know, but it had grown greatly in the interval. Ganelon may have been a fictitious character, though the rarely-used nominative of his name, Guénes, suggests Celtic origin. In any case, the four-horse execution is, as you stated, a common feature in folklore, and was attributed (fictitiously) to the "martyrdom" of St. Hippolyte, clearly out of confusion with the classical hero, Hippolytus, who was dragged by his chariot horses. St. Hippolyte became the patron of horses and horsemen thereby.

Every folk song or epic had its origin in a single orator/singer/poet, whether man or woman, humble or high. Most are heavily reworked over generations. In "Child Owlet", we may be unusually close to the origins. As I know next to nothing about most Scottish clans, I'll let someone else figure how the Erskines got in there.