Or HaChaim on Leviticus 1:9/אור החיים על ויקרא א׳:ט׳

1 את הכל. all of it. Torat Kohanim claims that the word הכל specifically includes the hooves and the horns of the animal. In view of this, we must ask why Abraham did not include the horns of the ram he offered in lieu of Isaac (Genesis 22,14), leaving the horns to be used as shofarot as described in Pirke de Rabbi Eliezer chapter 31. Perhaps we have to assume that Abraham did indeed offer, i.e. put the entire ram on the altar, and that due to the heat the horns פקעו מעל המזבח, cracked and fell off the altar; we learned in Zevachim 86 that once parts of the burnt-offering fell off the altar they do not need to be put back on.

2Another possibility is that the horns were separated from the ram prior to sprinkling its blood on the altar. Horns, etc., are not included as fit to be burned on the altar unless they are still attached to the body of the animal in question. If they have been detached, they have to be removed from the altar even if they had been placed on it already as we know from Deut. 12,27: ועשית עלותיך הבשר והדם על מזבח השם. According to Rabbi Zeyrah who stated in Zevachim 86 that the parts which had fallen off the altar are permitted for private use if they had been detached from the body prior to the sprinkling of the blood on the altar, one may fashion handles for knives from such horns.

3Another way of solving our question is that seeing the Torah had not been given as yet during Abraham's lifetime, the details recorded here did not obligate him. It is true that our sages in Yuma 28 held that Abraham carried out all the minutiae of the commandments. This included ערובי תבשילין, i.e. pre-cooking some food for the Sabbath following a festival on the eve of the festival in order to make preparation of food on the festival for the Sabbath permissible. I have already explained that Abraham did so voluntarily, in order to accumulate merits, not because he was under any obligation to do so. The same reasoning [that the Torah had not yet been given. Ed.] applied when Tamar married Yehudah her father-in-law, or when Jacob married two sisters while both were alive. The fact that the ram was stuck in the thicket by its horns was in itself a supernatural event; Abraham certainly did not have to burn up these very horns, a commandment applicable only in the future.