In order to understand what unclean animals are you need to take into account certain non canonical works, in particular the book(s) of Enoch.
What we can learn from the Bible is that God Did not make anything that was unclean - he can’t! And, we also learn what unclean means, and that is ‘mixture’. That is, unclean animals were not created - they are genetically ‘mixed’. This ‘genetic interbreeding’ occurred in the pre flood era. As said, the book of Enoch outlines this, and, it’s fascinating reading!
The dietary laws forbidding the eating of unclean animals came via the Law - but - use of unclean animals for sacrifices had always been prohibited - as sacrificed animals were to be blemish free. This was outlined, or rather documented, in Leviticus/Deuteronomy - but this requirement, as also that of the sacrifices themselves, was clearly taught.
Although we don’t see this teaching explicitly laid out pre wilderness, we clearly see the evidence of this when we read of sacrifices being performed right from the start of the Bible. And, it is the same for eating unclean animals. The reason for the 7 pairs was that the clean pure) animals were the ‘source’ for Noah’s needs. Again, this is referenced via non-biblical sources, although evidenced in practice seen in the Bible.