In Nathan's parable, what did the rich man take from the poor man?

Answer: His one ewe lamb

Rather than use his own flocks to feed a traveler, the rich man took the poor man's only lamb, which was like a daughter to him.

“but the poor man had nothing except one small ewe lamb that he had bought. He raised it, and it grew up with him and his children. It shared his food and drank from his cup; it slept in his arms and was like a daughter to him.”— 2 Samuel 12:3 (BSB)