The idea of an animal that can derive all or some of its energy needs from the sun is one that sci-fi authors have often touched on. Gert has proposed the nomenclature "bitroph" for such a creature, which I like. He also points out that certain sea slugs are known that "steal" chloroplasts from ingested algae and continue to use them as a sort of battery; they aren't necessary for survival, but help during lean times. Personally, I've always considered the way cold-blooded animals bask in the sun a form of photosynthesis and saw the skeletal adaptations of creates like Dimetrodon (such as sails and large fins) as potentially analogous to bitrophic behaviour.
Gert has also modelled a rough estimate "leaf" size for a mammalian-type metabolic rate. For an animal the size of a bandicoot, it would need to be a circle with a 40cm diameter. Obviously that would look a little ridiculous (and be extremely vulnerable to attack) but honestly it's well within the bounds of reality, which is a pleasant surprise. As he points out, the surface area would be reduced for less energy-intensive body plans, such as crustaceans, as well; plus, having large foldable flaps of photosynthetic skin isn't exactly a stretch (pun intended) when you consider bat and pterosaur wings. Interesting indeed.
Update 15/06/20: Gert's published a follow up. The short of it is that there are a lot more issues with the idea: available sunlight, metabolic need fluctuations (like movement), the fact leaves have mass. On the plus side, photosynthesis on Earth is pretty rubbish, so it could be made a lot more efficient.
