Her cat came up and rubbed itself against their legs.

itself 是反身代词，作 rubbed 的宾语。它指代与主语相同的事物her cat。当主语动作的接受者是主语本身时，要用反身代词，不要用宾格代词。比较：

Tom looked at himself in the mirror. 汤姆照镜子（看自己）。

 Tom looked at him in the mirror. 汤姆在镜子中看他（看别人）。