In non-Japanese media prior to Gen V (which includes Gen IV, when Arceus was introduced), the terminology of "mythical" was never used. This question is from 2015, after the distinction was made, but considering it wasn't really something people made a big stink of at the time, I'd wager that most non-Japanese people that grew up playing the earlier generations would have viewed "mythical" as a subset of legendary.
All of these "actually this Pokemon is a mythical not a legendary" answers are splitting hairs and arguing semantics instead of actually answering the question in any significant way.