On the sixth day, the earth gets its turn. Targum Pseudo-Jonathan on (Genesis 1:24) echoes the pattern already set in the sea: every living creature comes forth "the kind that is clean and the kind that is unclean" — cattle, creeping thing, and beast of the earth, each after its kind.
The Targumist is not adding a detail for its own sake. He is anticipating Leviticus. Long before there are Israelites to keep the laws of kashrut, the world is already structured with the categories that those laws will name. Creation has grooves built into it. When Sinai eventually comes, the commandments do not invent distinctions; they reveal distinctions that were there on day six.