Why aren't animals photosynthetic?
Why aren't animals photosynthetic on earth? And what would make it plausible for them to evolve to be?
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From the viewpoint of evolutionary dynamics, the reason why very few species are both autotrophic (photosynthesizing) and heterotrophic (hunting / foraging on other living organism) at the same times is that these two lifestyles tend to require quite different adaptations.
One notable example is motility. Efficient foraging organisms typically need to move, but movement consumes a lot of energy, which require a relatively high-intensity energy source, like consuming other organisms. Meanwhile, a photosynthetic organism rarely needs to move, but, with the low energy density available from photosynthesis, it also cannot afford to move much. Thus, in general, animals and other foragers tend to be actively mobile, while plants and other photosynthesizers tend to be sessile (or free-floating).
As a result, there are few fitness maxima between these two lifestyles. A creature with such a hybrid lifestyle can usually increase it fitness via mutations that improve its foraging efficiency at the expense of photosynthesis, or vice versa, and so evolution tends to drive such species (if they survive) towards one end of the spectrum or the other — either losing its photosynthetic ability to become a pure heterotroph, or becoming fully dependent on it, and thus a pure autotroph.
That said, among the countless species found on Earth, there are certainly some exception to this general rule, perhaps the most notable among them being Euglena and some related protists, many of which have functioning chloroplasts and practice both photosynthesis and phagocytosis.
The notable aspect of Euglena is that their mixotrophic lifestyle appears to be permanent, insofar as their chloroplasts reproduce within the host organism, and are passed down from parent to offspring. It appears that, for Euglena, this unusual lifestyle may be useful because it helps them survive and reproduce efficiently under variable environmental conditions, as they can switch between an active foraging mode and a passive, mainly photosynthetic mode depending on the relative availability of food and sunlight.
Another relatively common class of exceptions are kleptoplastic species, like the "photosynthetic sea slugs" mentioned in some of the other answers. These are animal (or heterotrophic protist) species that don't grow their own chloroplasts, but can harvest them from plants or algae they consume, retaining the functional chloroplasts (or, in some cases, whole algal cells) within their body for at least some time. Most such species are primarily heterotrophs, but the ability to retain harvested chloroplasts within their body apparently gives them some survival advantage, at least as an emergency energy / nutrition reserve, if nothing else.
Besides these living examples, there's also evidence that the evolution from a combined phagocytic + photosynthetic lifestyle to essentially pure photosynthesis has happened several times on Earth. In particular, the endosymbiotic theory of chloroplast evolution, which is nowadays all but universally accepted among biologists, states that all eukaryotic plants and algae are descended from an phagocytic ancestor that engulfed and retained photosynthetic cyanobacteria in a symbiotic arrangement, much like Euglena today. Eventually, adapting to a photosynthetic lifestyle, this hybrid organism lost the ability to forage for food, becoming totally dependent on its chloroplast symbionts for energy.
In fact, there's even evidence that this process was later repeated several times, with eukaryotic algae (with their own chloroplasts) being absorbed and retained by other phagocytic protists, most of which (with the notable exception of the euglenozoans, as noted above) in turn also evolved to become fully dependent on their photosynthetic endosymbionts, and lost the ability to forage for food. Typically, the algal endosymbionts then shrunk over time, as more and more of their functions were taken over by the host cell, becoming little more than simple chloroplasts themselves, but in many cases their cell membranes, and sometimes a remnant of their nucleus, remain and still reveal their origin.
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One possibility that has not yet been mentioned (actually inspired from githubphagocyte's comment on how the evolution of plants began in the first place): It could be that the animal doesn't do photosynthesis itself, but lives in symbiosis with a plant living on its surface. Probably not a macroscopic plant, but a single-celled one (so it basically grows in the skin).
A way how that could have been started: An ancestor animal is plagued by some parasite, say a fungus. Now it happens that an ancestor of the plant, which grows in larger amounts at certain places, effectively forming a green film on the ground there) produces a fungicide (to protect itself from other fungi) which also is effective against that specific parasite fungus. Because of this, that animal evolves the habit to rub its skin on the floor where those plants live.
Inevitably, this means that some of those plants will stick to the skin, which actually benefits both the animal (the plant sticking longer on the skin means better protection from the fungus) and the plant (because the animal will carry it to new places to grow at). So over time, it will be perfectly normal that those animals will be coated with plants, although there's not yet any other connection.
However, as the plants adapt to living at least part time on that animal's skin, they may evolve to draw water out of the animal's skin (which allows them to survive longer periods on the animal). As long as they don't draw too much water, the fungus protection the plants provide will still be more benefit than the extra water the plants draw from the animal (especially if the animal has no problems finding more water to drink).
However now the plants will be independent of eventually getting on the floor, as they can easily live just on the animal alone; transfer from one animal to the other can happen due to the animals rubbing their skin on each other (which may happen a lot for social animals, and will inevitable happen for young mammals drinking their mother's milk). Therefore the plants may over time lose the ability to grow on the floor (where they have lots of competition from other plants, which they don't have on that animal's skin). In that process, the plants will likely also develop stronger connections to the animal's skin.
Now the plant cells will probably develop a way to transport water and nutrients between them, because the cells entering the skin (and this tapping the water) will not have as much exposure to sunlight, while the outer cells will be exposed to the sun light, but not have a good water source. Now since inter-cell transport of photosynthesis products from the production places on the surface to the lower cells living in between animal cells happens, it is not unlikely that the animal will develop also a way to tap that nutrient resource for its own metabolism.
At this point the symbiotic "plant-animal" is complete.
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