As Rashi points out, the Talmud held that the animals involved here were created “in their full stature” – that is, as adults.
Rab Judah further said: The bullock which Adam sacrificed had but one horn in its forehead, as it is said: And it shall please the Lord better than a bullock that hath horns [makrin] and hoofs’.4 Butdoes not makrin imply two horns? — R. Nahman said: Mkrn is written.5 Rab Judah further said: The bullock which Adam sacrificed had fully developed horns before it had hoofs,6 as it is said: ‘And it shall please the Lord better than a bullock that hath horns and hoofs’; the verse first says: ‘that hath horns’ and then ‘hoofs’. This supports R. Joshua b. Levi, who said: All the animals of the creation were created in their full-grown stature, with their consent, and according to the shape of their own choice
Targum Onkelos: And the Lord mad, the beast of the earth after its kind, and cattle after their kind, and every reptile of the earth after its kind; and the Lord saw that it was good.
Rashi: And… made: He fixed them in their complete form, and in their full stature. — [from Rosh Hashanah 11a, Chullin 60a]
Rambam: By in-formation God generated the carnivorous animals according to their kinds, the herbivorous animals according to their kinds, and the animals which travel using their whole bodies according to their kinds. God confirmed them in their existence in God’s will.
Ibn Ezra: God made the wild beasts of the earth according to their kinds, and the domesticated animals according to their kinds, and all the small animals according to their kinds. God understood that it was good.