They can be found. Frogs are a bit harder. The old wiki page said rats only come out at night, but pretty sure I've seen them at all times. They are only on certain biomes, but as I don't use a custom client, I can't tell you which those are (all are forest types that I can tell).
This might help:
http://ringofbrodgar.com/wiki/FEP_Table Sort by the Energy recovered column.
If you use rat on a stick as a base for best easy food (225 energy/hunger), then you also have perched perch (needs onions) and a couple others that need a frying pan once you get a bit of iron.
If you have a bit of farming, there are several foods that are as good or better than frog legs. You might need to do a bit of light hunting. Bats aren't too bad, and squirrels, rabbits, and chickens count for any meat.
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