It's common for long-running shonen series to be plagued by a few inconsistencies, giving way to a few retcons along the way. However, none come close to the overarching nature of one retcon pertaining to the tailed beasts that take center stage in Naruto Shippuden. While it was implied that Tailed Beasts were an ever-present aspect of the series, the reality is actually quite the contrary as the notion of nine different Tailed Beasts did not exist in part 1 of the series.
One-Tails and Nine-Tails were certainly part of the initial story, but these two behemoths of chakra were simply considered monsters or demons, not sentient beings labeled as Tailed Beasts. Several instances throughout the series prove how the Tailed Beasts were never an initial part of the narrative, rather they were retconned into the series to further the Akatsuki plan and the show’s overall premise.
No Mention of the terms 'Tailed Beasts' or 'Jinchuriki' in Part 1
One of the most obvious telltales of the Tailed Beast retcon is that part 1 of Naruto has zero mentions of the terms ‘Jinchuriki’ and ‘Tailed Beasts.’ The concept was nowhere to be seen, with most just referring to these chakra beings as demons or spirits. While Shukaku and Kurama were the only Tailed Beasts in part 1, the two were never referred to as One-Tails or Nine-Tails. Right from the first arc of Shippuden, however, these terms suddenly became apparent without any sort of exposition.
Furthermore, Shippuden constantly reiterated that the Tailed Beasts were never referred to by their names, with most people relegating these creatures to names based on the number of tails they possessed. However, this is incongruous with part 1 where One-Tails was constantly being referred to as Shukaku, clearly denoting the fact that this concept was non-existent before Shippuden.
Intel About the Akatsuki's Plans Was Vague
Another indicator of the Tailed Beasts being a new concept to the series is Jiraiya's Intel about the Akatsuki when he returns to the village in part 1. In his conversation with Kakashi, he states that the Akatsuki are on the move and might target Naruto for the demon fox sealed inside him, with plans to form an inexplicable jutsu with that power. It's understandable that the author would withhold information from the viewers, however, the series was at a point where it would have made no sense for this information to be withheld considering it was discussed so openly right after in Shippuden.
Shippuden laid down most of this information right at the beginning, yet the source of this information was never revealed, simply implying that it was already common knowledge. Moreover, Jiraiya and Kakashi were two of the most intellectual shinobi in the entire Hidden Leaf, yet they failed to come to the conclusion that the Akatsuki could be after other Tailed Beasts if Kurama is a clear objective.
The two did come to the conclusion in Shippuden, though it was implied as if the audience would already know that this is the case rather than it being an entirely new piece of information. The contents of the Akatsuki's plan also became widely known among the upper hierarchy of the Hidden Leaf, something that was previously kept confidential. This incoherence is most likely due to the Tailed Beasts being retconned in Shippuden, as the concept had not yet been concocted in part 1.
One-Tails and Nine-Tails Are Based on Japanese Mythical Beings
Many argue that the Tailed Beasts were a planned element from the beginning of the series by pointing towards Shukaku and Kurama being present right from the start. However, a closer look at the mythologies of these two creatures reveals that they were simply meant to be creatures that were inspired by Japanese folklore. The Nine-Tails Fox is a fox spirit famous in Japanese mythology, while Shukaku is based on Tanuki or Japanese racoon dogs – another element of the culture’s mythology known as Bake-Danuki.
The number of tails conveniently fit into the narrative of the nine Tailed Beasts, making the addition of the other beasts much easier. Shukaku's underwhelming strength is another indicator of One-Tails simply being a demon and nothing more. Gamabunta was able to easily subdue the beast, with Naruto using just a morsel of the Nine-Tails chakra. If Shukaku were to be a Tailed Beast from the beginning, his strength would have been somewhat comparable to the other Tailed Beasts, yet he was merely presented as another mythical creature. These instances clearly demonstrate how the Tailed Beasts were a later addition, albeit one that took the series’ premise to an entirely new level.