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This question is simple to answer. As defined by wordweb, the most famous downloadable english dictionary (maybe), it states that Beavers are large semiaquatic rodent with webbed hind feet and a broad flat tail. This only fits the characteristics of Bibarrel. Look at how Bibarrel looks like. On the other hand, mouse is described to be rodents typically resembling diminutive rats having pointed snouts and small ears on elongated bodies with slender usually hairless tails. The hairless tails, fits our first understanding of why Bidoof is a mouse. Second, Rattata has buck teeth right? And is a mouse pokemon right? Therefore, if you were to say Bidoof is a beaver, I would say rattata is a beaver too.
Rattata>Mouse pokemon>Bucked teeth, short tail hairless (althought rattata doesn't have short tails but mouse have long thin tails too)>Bidoof
Beaver>Bucked teeth, flat elongated tails, semiaquatic>Bibarrel (normal, water)
And for goodness sakes, the pokedex is made by game creators. It is not a sentinent device in the virtual world that gives you explanation on a pokemon based on its own thoughts. The creators intended it to be a tiny mouse based on a different intution. They wouldn't just create crap. If you think so, then why is Giratina considered a Renegade pokemon when it looks more like a serpent or why is skiploom a cottonweed pokemon when it does not produce cotton? If you consider what the creators have done, you will understand their work.
The pokedex is not nonsensible, you just need to understand biology rather than judge things through your intution.
ANYWAY THANKS FOR THIS POST, I COULDN'T HAVE UNDERSTOOD MORE ABOUT THE DIFFERENCE BETWEEN BIDOOF AND BEAVER.