A lot of early anatomical work was performed on animal corpses, particularly dogs. I can't speak to how much human-derived anatomical knowledge was claimed to be from animals, but it is worth noting that that misconceptions about human anatomy dating from Roman physicians and authors like Galen, who worked on dogs, were still present in 18th and 19th century medicine; classical medical texts were revered so highly that misinformation in them was accepted as true even as information from human dissection became accessible. Source: Of the Greatest Benefit to Mankind, by Roy Porter.