I heard from a friend, then found at least one corroborating quote that leads me to believe Pope Gregory I declared Laurices/Rabbit Fetuses to be fish/marine life?
Where did this notion come from?
That's an interesting claim, where did you find the corroborating quote? I assume you're talking about the issue with the Catholic practice of abstaining from meat on select days.
While you wait for an in-depth answer from a fully qualified historian, perhaps this post here might be of interest. It details the development and the practices in question, and the allowances and interpretations of it across time.
u/Loknik
Cheers.