I don't think that the concept of "legitimacy" itself is a good one for English speaking countries, because one can legally name a child anything one wants in those countries (except perhaps New Zealand.) Therefore all names are legitimate.
In countries which have laws which limit the names one can give, you can perhaps use "legitimacy" as a relevant concept. But in the USA and the UK, all names are "legitimate". Whether they have ever been used in the past for real people is an interesting question, but that they never have been does not reflect on their "legitimacy".