Honestly while we don't actually know I have no idea how anyone could actually think the third sacrifice would just be like, regular animals and such.
Like on a very basic narrative level making it the Convocation just wanting to use literal cattle undermines the themes of both ShB and EW massively. It doesn't give the story shades of gray or anything close to that.
The convocation wanting to sacrifice new sapient life because they want back what they lost vs Venat sundering her own people partly in defense of that new innocent life is a compelling and interesting narrative element.
Having her do it to protect literal chickens makes both her and Emet look like idiots. Like why the fuck would Emet beat around the bush so much with us if the original end goal was something that simple?
Regardless of who you like more making it literal animals would just make both sides look really dumb, not tragically compelling.