I'm sure he must have had a life before Zagato, even in the anime. I seriously doubt that he just popped into existence when Emeraude wanted to give Zagato a companion to be there for him the way she could not. (My headcanon for this fic says that Emeraude went looking for someone or something to fill that role for Zagato a month or two after Lantis left, because Zagato seemed so lost without his brother.)

I hadn't intended to even mention anything about it in the fic, but that line about him being able to use a sword just popped into my head and wouldn't go away. So then I had to have Zagato ask how he knew that, because seriously, how does a spirit with an animal form learn to use a sword? And that naturally led into developing some backstory for him so I could give Zagato a reasonable answer.