Apparently pigs can be possessed with sprints. I always assumed that was why they were considered unclean.
From Mark 5: So the demons begged him, "Send us among the pigs, so that we can go into them!" So he let them do this. The unclean spirits came out of the man and went into the pigs
Logic has no merit when you're talking about the supernatural, because the rules can be whatever you want. Maybe animals can't be possessed by spirits because God only gave "true souls" to humans, and possessing spirits can only replace "true souls." Maybe they require a brain with a certain complexity which only humans possess. Maybe spirits just think it's gross, like bestiality.
Is the above statement true or false? It's about the supernatural, albeit indirectly, so we can't be sure shrug.
I'm interested to know why these were picked out for prohibition.