In this thread, we discuss theories regarding the development of Pokemon games. All generations allowed! I'll start.
>Arcanine was originally meant to be Normal-type
This may seem like a stretch, but bear with me.
>Arcanine's japanese name is "Windie"
>It's beta name was "Wing"
>It's running is compared to flying
>Flying-type moves in gen 1, save for fly, are all associated with birds in their Japanese names (Mirror Move is Parrot Mimicry, Sky Attack is God Bird, etc.)
>An unused "bird-type" exists as leftover data of the Gen 1 games, which share the same properties as flying-type does now.
>The only wind-based moves in the game (whirlwind, gust and razor wind) were all normal-type at the time.
>Arcanine was originally paired with the legendary birds, which already included a fire-type
>Flying-type probably wasn't created until Gamefreak realized that birds weren't the only creatures who could fly.
My guess is that it got changed to fire later on in development because there weren't enough of them in the game.