>>44277030I have no source, other than the "mouse pokémon" and here´s a couple of things I think happened.
1.-Gen 1 had Charizard who is a Dragon (Typing or not, he is based on one)
2.-Gen 2 had an abstract looking fire starter who is some sort of mole-rat that transforms into a big ass non-specific beast with fire spikes on the back
3.-Gen 3 had a fighting chicken
4.-It is in GEN 4 that they probably realized a Monkey could go well together with the other two starters, and someone at GF probably noticed the previous 3 pokémon could fit into the Chinese Zodiac, not only that but Infernape was given chinese deisngs and is also based on Sun-Wukong.
5.-They followed the Chinese thingy now a lot closer since Emboar also has similar body designs to Infernape, in Sun-Wukon story, he also has a pig friend
6.-By this point, they probably did not want to do more fire fighting starters, but probably decided to keep the Chinese Zodiac theme to a degree while also keeping in line with the other 2 starters each gen, that why Fenekkin is not exactly a "dog" but in that same regard, Charizard is not exactly a Chinese Dragon, but they are just inspired by it.
Likewise, there was an interview with Masuda I think back in gen 5 when he talked about the starters there and that he had a lot of trouble with Oshawott´s final evo because he just couldn't find a way to make an Otter look "menacing" or cool or whatever the fuck, he went to the zoo and saw the sea lions, who of fucking course, are NOT otters but they decided to base the final evo off those, you can probably also say the same happened with Cyndaquil, while not exactly wanting to keep a "chinese zodiac pattern" they maybe started with a mouse and eventually realized that an odd mix of a mole, echidna and a bear could look way better for a 3rd stage than keeping up with the mouse theme there.
tl;dr There is a pattern that maybe started as a coincidence and from GEN 4(3) onward they decided to stick to it.