In Genesis 7, God tells Noah to take with him 7 pairs of clean animals in contrast to only one pair of all other animals. Additionally, 7 pairs of birds were to be taken as well.
Take with you seven pairs of all clean animals, the male and his mate, and a pair of the animals that are not clean, the male and his mate, 3 and seven pairs of the birds of the heavens also, male and female, to keep their offspring alive on the face of all the earth. Genesis 7:2-3
However, prior to the flood, it seems that mankind was not allowed to eat meat, as God specifically allows for it after the flood. Furthermore, the distinction between clean animals and unclean animals for consumption was not revealed until the time of Moses.
Every moving thing that is alive shall be food for you; I give all to you, as I gave the green plant. Genesis 9:3
I guess it is possible that people did, in fact, eat meat prior to the flood, but did so against God's decree. However, not many of them were following God's decrees.
So, is there any indication of what a clean animal would have been right before the flood that distinguished them from other animals?