How did a ram suddenly appear out of nowhere? What made Avraham realize that it was there for him to sacrifice in place of Yitzchak?
Rashi and Targum Yonasan: The ram had actually been there since the six days of Creation, 1 but Avraham only noticed it the moment the angel stopped him from Shechting Yitzchak. In fact, Rashi explains that the ram was running towards Avraham, and the Satan caused its horns to become entangled 2 in the thicket.
Rashbam: One minute Avraham saw the ram walking past, and a moment later 3 , it was caught in the thicket. So it must have been the angel that caught it, for him to bring instead of Yitzchak.
Seforno: Avraham realized that the ram was there miraculously, since one moment it was not there, and the next moment it was. 4
What is the significance of the words "Tachas B'no"?
Rashi: To teach us how, at every stage of the Avodah, Avraham declared 'May it be considered as if Yitzchak was Shechted ... as if blood was sprinkled ... as if his skin was stripped ... as if he was burned and turned into ashes!'
Why does it say that it was "Ne'echaz bi'Svach b'Karnav"?
Da'as Zekenim, from Pesikta: Hashem hinted that if Yisrael will be caught in Aveiros, they will blow horns (Shofaros) and Hashem will recall the Akeidah and vindicate them in judgment.