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Cyndaquil's Japanese name contains a pun on "porcupine" as it is partially based on one.
Porcupines are rodents.
The Japanese language does not distinguish between mice/rats/rodents and are considered one and the same (nezumi) regardless of scientific classification.
Therefore, as far as Japan is concerned, Cyndaquil is a mouse/rat/rodent.
It is featured on "Year of the Rat" promotional material.
There is simply no argument against this.