They thought she was named for a star. An easy mistake to make, since everyone else in her family was named after stars or constellations. And to her parents' credit, one of the other names for the star people thought she was named for was 'Al-Najid', 'The Conqueror'.
No, Bellatrix LeStrange (nee Black) was named for something rather different. A cousin of her mother's, who had a small talent for Divination, had told Druella and Cygnus that their eldest would be 'a symbol of purity, and yet a fighter'. And a magical test on the day of her birth showed she had the potential to be an Animagus, and her possible form.
If one were to speculate on what Bellatrix LeStrange's Animagus form would be, it would be a safe guess to say that the creatures that spring to mind most readily would be either fearsome, noisome, or deadly in one or more ways. The Death Eaters she served with had a pool, back during the First War, that her Animagus form, if she had one, was a rabid skunk. When this was mentioned, her husband Rodolphus and brother-in-law Rabastan simply smiled beatifically before subjecting the person who mentioned it to the Cruciatus.
The truth of the matter was in fact quite different, and wholely unbelievable. Who would suspect that Bellatrix LeStrange, the most feared liutenant in the Dark Lord's service, the woman responsible for torturing the Longbottoms into insanity, would have the Animagus form of a pure white mare?
They thought she was named for a star, the third-brightest star in the Constellation Orion. She was named for the Celtic goddess of horses, in her aspect as worshipped by the Romans, a patroness of cavalry. She was named for Epona the Warrioress.
Epona Bellatrix.