Even though no prior description of clean and unclean animals had been recorded, God had made that distinction during the days of Noah. Two of a kind of unclean animals and seven each of clean animals boarded the ark at the time of the flood.
As God continued instructing Moses and Aaron regarding life and conduct, He gave them specific descriptions of each clean and unclean species. There were clean and unclean animals of the land, air and waters included. Eating or even touching an unclean animal caused the person to be unclean. If a carcass or part of a carcass touched a person or object, that person or object became unclean.
God had His reasons for labeling various animals unclean. Whether one understands the purpose of various commands is not important. The important thing is to obey.