When Chris Claremont took over the X-Men after the relaunch in 1975, he was left with a team of six mutants, since Beast was off being mutated and whatnot, two of which weren’t realllllly X-Men, but just kind of hanging around the school for kicks. One of those was Havok, the other was Lorna Dane.
But up until that point, Lorna Dane didn’t have a code name. She had the whole Magneto-robot storyline gone down, but she was left with no name. So Claremont gave her one.
So there you go. Lorna Dane becomes Polaris, and the name sticks. And that is just one more reason why Chris Claremont is awesome.