Okay I updated the answer. Some of these categories do not have consistent rules. I have tried to make common-sense choices:
* Fletchling is as much Paldea's regional bird as Wattrell, but I think it's close enough.
* Lechonk is very obviously the intended addition here. Zigzagoon and Yungoos aren't rodents either.
* I added dogs because the trend has continued for another two generations. Gen 6 is just an outlier. I personally think dogs/cats is what the developers have intended due to Shinx and Litleo, but then you also have to add Meowth, Purrloin, etc.
* I didn't include starters because that's obvious, and I took out legendaries for the same reason (and also because Mewtwo for Gen 1 stretches the definition).
* I didn't include fish because I don't think the trend is strong enough. Magikarp and Feebas are clearly connected, but the others can be chalked up to coincidence.