Sacred and unclean animals

[Brian37]

In ancient Egypt pigs were considered as sacred
In Karnataka, India Serpent deities are worshiped
In Hinduism cattle’s are considered to be holy
In Islam and Judaism the pig is considered unclean
Who decides these things, if a new animal was discovered in the deepest Amazonian forests, who would decide if was just another animal, sacred or unclean, what tests would be used to decide this?

Since it isn't scientific, I'd say the person or people who started those myths in antiquity decided and successfully sold those myths.

I would say however, those gap filling avoiding certain animals might have resulted in seeing people get sick from not fully cooking certain animal meat and had no clue what bacteria or toxins were. They might have come to the false conclusion that the animal had magic powers over their lives and thus exalted the animal as being something to protect. 

But just like holy clothing it is simply another superstition humans make up and buy and sell as a form of gap filling.

There are real health reasons you avoid certain foods and why you cook raw meat. But no, neither pigs or cows have any magic ability to affect where you go in an after life that does not exist.