I am assuming that Trump calls Elizabeth warren Pocahontas because at some point in the past she made reference to her Native American ancestry.
She did. From the Boston Globe:
Democratic Senate candidate Elizabeth Warren acknowledged for the first time late Wednesday night that she told Harvard University and the University of Pennsylvania that she was Native American, but she continued to insist that race played no role in her recruitment.
“At some point after I was hired by them, I . . . provided that information to the University of Pennsylvania and Harvard,’’ she said in a statement issued by her campaign. “My Native American heritage is part of who I am, I’m proud of it and I have been open about it.’’