>>49577759I wouldn't have called the original three therian forms the four symbols until Enamorous
Since fuujin and raijin were just a pair, Landorus, with its inspiration and its role as leader, is a third wheel to Tornadus and Thundurus, aside from the differences in movepool, stats, abilities, etc. It was much easier from this 2+1 perspective to just call Tornadus and Thundurus fenghuang-dragon as supposed to the azure dragon and vermillion bird, since the former group is a natural duo as supposed to needing a third/fourth member, plus the former group's colors match up better compared to the latter's (Tornadus was originally red, yes, but Landorus not being white didn't help, and the color of their navel gems mismatched too).
But now with Enamorus, Landorus doesn't have to play a third wheel now that we have a fourth wheel. We have a male-female fertility god duo that, together with fenghuang/dragon, form the four symbols.