Mosiah 25:12

And it came to pass that when those which were the children of Amulon and his brethren, which had taken to wife the daughters of the Lamanites, they were displeased with the conduct of their fathers, and they would no longer be called by the names of their fathers; therefore they took upon themselves the name of Nephi, that they might be called the children of Nephi, and be numbered among those which were called Nephites.

The children of Amulon and his brothers didn’t like what their fathers had done. Their fathers had married Lamanite women. These children didn’t want to be named after their fathers anymore. So they took the name of Nephi. They wanted to be called children of Nephi and counted as Nephites.

Now the children of Amulon and his friends—the ones whose fathers had married Lamanite women—were ashamed of what their fathers had done. They refused to be called by their fathers’ names anymore. Instead, they took the name of Nephi so they could be counted as Nephites.