I think you think I don't believe you, but that's not the case. You saying that the name is the result of the actual author having done all the English themselves makes more sense to me than my initial belief that it was a mistranslation, and it works better as an explanation.
Because knowing what I do about the general state of English knowledge among many mangaka (it's not great) vs knowing that you generally do a very good job with translating into English (I'm pretty sure it's not your native language...? Which actually makes it more impressive) it is more sensible to me that the Japanese author would potentially have decided to name the dog after King Arthur (being a famous name in English folklore) and then reverse-engineered the katakana, which google tells me is アーサー for "Āsā". Lacking a strong phoneme equivalent to the short "u", and the Japanese-used long "u" being kinda ungainly in this situation, it gets warped into "ar-thar". Though weirdly Google translate does render アーサー as "arthur" if you ask for the English translation. So maybe there was no translation help and the author used a manual dictionary with enough knowledge to figure out the "s" was a substitute for a "th" and the long "ā" were meant to be "ar" but didn't think to find the proper English spelling of the name.
It makes more sense that you left it as is, preserving the original mistake than it does that you translated it wrong and arrived at a somewhat odd name choice.
Apologies if I was unclear.