I was challenging the Black Tower and competed against a boss. Her second Pokemon was Tornadus! How is this possible?!
It is possible becuse Game Freak wanted it this way, we cant answer this question.
Also, this guy has an Entei and a Articuno, this guy has a Latios, and this girl has a Heatran, a Latios and a Suicune, also because Game Freak wanted it that way.
That's all. Gamefreak just decided to give her a Tornadus.
Other bosses have Pokemon like Heatran and Latios as well.