This
Magearna was created over 500 years ago by a man named
Nikola to contain the power of the Soul Heart, a weapon that could be used to protect the kingdom of Azoth. After a war broke out, it, in it's Poke Ball Form, escaped to a field far away where it met Volcanion. Over the 500 years, it's paint chipped away to where it became it's current Form, and Volcanion always protected it.