Sforno on Genesis 25:24/ספורנו על בראשית כ״ה:כ״ד

1והנה תומים בבטנה, before they were born, the onlookers already realised that there were twins in her womb. Therefore, when the first one emerged being completely covered with hair, this was a birth which was presumed to be more difficult than the passage of Yaakov whose skin was smooth and did not cause friction with the lining of the womb, they called the first one עשו, attributing his easy passage to his twin brother who had helped push him out of the womb of his mother. [I believe that the author understands the plural ending in עשו, as equivalent to assu, i.e. the result of a combined effort by both him and his brother.]