It all depends on the context. The past perfect is appropriate when describing that, at some past time, the snake was lethargic as a result of having previously eaten a frog. For example, "Last year I bought a snake.
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vishaljain23231On the time scale, past perfect comes before past tense. So, since snake swallowed first and then it became lethargic, I think the correct way of paraphrasing would be "After the snake had swallowed the frog, it became lethargic.I am no expert! Just an opinion.That's what I was thinking.