I asked the I earlier where the I heard of Feteri. When the I didn't answer and spoke of Teferi instead, I just assumed you made a spelling mistake.
I don't think that he was ever called Feteri, I have never heard that. Maybe the I can ask who told you that and they can give you more information, I know nothing about it.
I have heard that people mistakenly thought that TafarI meant Creator, instead of "One to be feared", but I didn't know that Feteri was the Ethiopian word that meant Creator.
Every english text I have seen says TafarI. Including the ones where the translations were sanctioned by Haile Selassie I
Haile Selassie I