The only hint of this in Gen 1 is it's category ("Legendary") but these appear to be more like descriptors of the Pokemon, rather than classifications (the other Gen 1 Legendaries do not have the category "Legendary").
The Pokedex entries up to Stadium are as follows.
Red and Blue: A Pokémon that has been admired since the past for its beauty. It runs agilely as if on wings.
Yellow: A legendary Pokémon in China. Many people are charmed by its grace and beauty while running.
Stadium: A Pokémon whose beauty is legendary in China. It is said to run gracefully and lightly, as if it were flying.
It seems like there are two possible explanations.
- It was not intended to be Legendary, but the makers of the anime misunderstood the category and so included Arcanine in with the actual Legendaries (the Birds and Mew/two). In Yellow onwards GameFreak made sure to clarify that it was more like a historically worshiped/popular Pokemon in China, rather than an actual Legendary.
- It was originally intended to be Legendary, but this was removed from Gen 1 at the last minute and remnants of this can be seen in the anime.
Do we have any confirmation of either of these explanations from GameFreak/Nintendo/the makers of the anime? Or is there another explanation that has evidentiary support?
This question is about the original intent of Arcanine, not why it's included with the Legendaries within a specific shot of Pokemon Origins. The accepted answer to that question simply points to an earlier shot of the anime, says "to match this" (which is technically correct) and then reiterates the claim that my question is asking about. My question is about the claim itself and whether any support for it exists.