That’s a very, VERY good question.
Perhaps there was an early oral tradition about clean and unclean beasts,
even before the Law was given to Moses.
(( Or, and I hate to say this,
but the whole question may be moot.
Why?
Most Catholics that I have ever heard, no longer believe that
a global deluge, or even a local (with a VAST local area, though) flood,
ever occured, and that the whole story is a morality tale emphasizing
that God will punish sin. Even my quite conservative priest holds this
view.
Of course, this creates problems that those who hold this view,
perhaps do not realize.
No flood means no Noah. No Noah means no rainbow covenant
and NO Noahide laws,
just as NO LITERAL Adam and Eve means NO original sin,
and also means that St. Paul was dead wrong when he asserted that
“through one man ((ADAM)) sin entered into the world and death through sin.”
I did not intend to go off on this tangent,
but this whole “new catholic” re-interpretation of Genesis chapters 1-11
in favor of the evolutionary world view, has disastrous consequences that
cannot be fudged over. ))