Genesis 7:2 ESV
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,
Both clean and unclean animals were gathered into Noah's ark. This might be a foreshadowing of those who will inhabit the new earth.
Nothing unclean will enter the New Jerusalem when Christ/God come to the earth, but that doesn't mean there is nothing unclean beyond the gates of the city. Not all the earth dwellers will receive the number of the beast.