Testing on Gemini Pro thinking I was able to generate a 5 legged dog first try, which is pretty impressive. Pasting that into a new session it counted 4 at first, but when I asked it to count more carefully it came back with 5 (and a detailed description of the positioning of each limb). Interestingly it suggested the anomaly could have been caused by AI! This is a good test I think - we seem right on the threshold of it being doable.