As far as Adam and Eve knew, when God began to question them, they were dead in their sin, which in actuality they were. They had followed the prince of the power of the air and the passions of their flesh. They could not have known there was a mercy option for them; it had not been revealed, even though the option existed from all eternity.
God, who is rich in mercy, with the great love with which he loved these two persons who were naked before him in sin, made them alive with the death of an animal that provided their skin-coverings. By grace they were saved, and their guilt and shame before God was removed. They had nothing of which they could boast. They were the Creator’s workmanship alone. Their story is paradigmatic of salvation throughout Scripture.
I think the slain animal might have been a lamb. I can’t be certain, but in light of Gen. 3:15, it makes good sense.