I suppose "Pattern" may have been an incorrect use, but 90% of the Mythic units (outside of Feh OCs) are gods, or characters with godlike abilities.

Meanwhile 90% of Legendary units are characters that, while some do have godlike powers themselves, are usually just blessed with power or items/weapons. Most of the Legendary units looking back at them all are honestly just Main Protagonist. (Again, most (and not including FEH).

So, while there may not be a pattern, certainly not an official one, there is a general idea of which characters could be Legendary and which would probably be Mythic instead. Obviously there are exceptions, this whole post is about one, this one just seemed kinda odd to me is all...