If we're going hard science, we would have to imagine some kind of biology which evolved from the beginning to use lightning energy. It's hard to imagine an animal acquiring that talent from a different evolutionary line without getting fried.
So let's imagine how a single-celled organism might use lightning.
On a planet with the right chemistry, and a lot of lightning, there would be ions in the atmosphere or in the ocean. These ions could be used as a kind of energy source. A multicellular organism might evolve a membrane to selectively acquire these ions and use them to build up more complex organic molecules.
Could we imagine this kind of organism making the leap to getting energy by being shocked? Maybe! If it already had efficient ways of moving ions around, maybe it would be somewhat more immune to a lightning strike. And that gives it an evolutionary path to being totally unharmed by one, and then perhaps seeking them out. Then your organism would be collecting raw elements from the environment and seeking lightning to convert them.
I'm not a chemist or biologist, but maybe looking at the biology of electric eels will be instructive.
The real problem is that a lightning strike is so unpredictable that your organisms aren't very likely to ever get hit, no matter what they do. Unless...
Maybe, they can float like balloons, and they can link themselves together in a long line, like an ant bridge. Then they can form a wire - connecting different layers of the atmosphere - with a much greater chance to have energy flow. Perhaps they really evolved to exploit energy differences between layers of the atmosphere, and the lightning is just a neat trick they do sometimes.