My money is on the monks/priests. Not all were vegetarian, but there were some very powerful ones that still are (like Zen Buddhists). They influenced writing a lot. And, like any other religious person, they were incredibly clever at circumventing their religious rules. So they defined beavers as fish so they could eat them and to me them defining rabbits as birds so they could eat them (at temples where they already defined birds as not really animals so okay to eat) seems pretty plausible to me.
These keep being fun, thank you!