Before I posted this, I quick googled searched to see if sea otters were rodents and saw an answer saying "yes" so I assumed it while questioning its authenticity.
In the wake of this error I thought that maybe since Chespin is based off of a Hedgehog and Cyndaquil is based off of an Echidna both alluding to Sonic and Knuckles , the third variation could be a Water-Type Porcupine or something belonging to the same family of animals, which I don't know if they are but they look similar.
I see. Whoever said that otters are rodents need to get their cookies taken away from them, because otters are as close to rodents as pigs are to humans. They're small, fuzzy, and cute, but they are definitely not rodents. (For the record, weasels and other mustelids are a clade of
Caniformes, meaning their closest relatives are dogs and doglike creatures, such as bears, skunks, raccoons, and badgers.)
Cyndaquil could definitely be based on an echidna, which would be interesting. Echidnas, for the record, are monotremes, a clade of marsupials who lay eggs. Their closest relative is the platypus, the only other non-extinct monotreme. Since they're marsupials, the next closest relatives are kangaroos, wallabies, koalas, wombats, and opossums. (Porcupines, hedgehogs, and echidnas all having backs covered in still hard quills are an example of convergent evolution, where the same or similar adaptations have evolved independently of each other. Carnivorous plants are the widest example of convergent evolution, with at least 115 instances of plants having evolved a means to catch animals, often in the exact same way.)