A new mystery I happened across!
In USUM, if you showed
Shigeki Morimoto in the GF Offices a Piloswine he'll say this:
"You know Piloswine? I designed that Pokémon. I felt like I had to get all 12 animals in the Chinese zodiac together, you know?"
In addition to revealing he designed Piloswine (as well as Shuckle, though only trivia he gives on it was it making Berries into Berry Juice in Gold & Silver), he also revealed that, in the eyes of GameFreak,
the creation of Piloswine meant every animal in the Chinese Zodiac had a Pokemon equivalent.
Now we all know about the theory that GF are basing the Fire-type Starters on the Chinese Zodiac which has evidence both supporting and denying it. I bring this up because one thing people use to disprove it is that Cyndaquil and Fennekin don't quite fit with this theory though a counter argument to that is Pokemon take artistic liberties about what Pokemon represents what animal and they often do "close enough", so Cyndaquil represents the mouse (even though its a hodgepodge of animals such as echidnas, shrews, and badgers; though its called the "Fire Mouse" Pokemon) and Fennekin represents the dog (even though its a (fennec) fox, but foxes are canids like dogs). I feel that last point is worth mentioning as, going through the Gen I & II Pokemon looking for the Chinese Zodiac, there are some you need to take with a squint. So, let's get to it:
Rat: Easy enough, Ratatta and Raticate. Maybe Pikachu if you count it being a cousin rodent, the mouse.
Ox: While not an exact match, Tauros is a bovid similar to an ox. Should be noted that in some places the Ox is replaced by a Cow, which not only fits Tauros (bulls are male cows) but also Miltank if it was created at that point.
Tiger: Here is where things get tricky. Gen II does have a "tiger" Pokemon, the sabertooth tiger Legendary Raikou. This would be interesting as that means Raikou was created before Piloswine. But could there possibly be another Pokemon which may not be so direct? Possibly two. First, if they just focused on any feline, than Persian could be an option. While not a tiger, and more like a house cat, it is a more fiercer looking cat that could take some traits fro jungle cats like tigers. Second, if its the appearance of a tiger they focused on, than it could be Arcanine. Arcanine is based on the Legendary creature the Shisha which is a combination of dogs, lions, and tigers. Though if it is Arcanine that means Arcanine can't be the representative for the dog (not that animal doesn't have other good options).
Rabbit: Another tough one. We don't exactly have a rabbit like Buneary or Bunnelby just yet. One possible choice is Nidoran or Nidorino/Nidorina. Its easy to overlook because of their final stages but they do have the general shape and long ears of a rabbit. Another possibility is Azumarill. While Marill looks more like a mouse, when it evolves its ears grow long like a rabbit (and it's also called the "Aqua Rabbit" Pokemon).
Dragon: Now I know what you're thinking: Dragonite, or maybe even Charizard. But let's remember what a Chinese dragon looks like, it's more of a serpent. So Dragonair? Not a bad choice either, but the Asian dragons all look much fiercer. A fierce looking serpent, ring any bells? Gyarados! Gyarados is based on the tale of carps jumping over a waterfall and turning into a dragon (and it was supposed to be a Water/Dragon-type).
Snake: Ekans and Arbok, next!
Horse: Ponyta and Rapidash, next!
Goat/Sheep: In case you're wondering, the original word for the animal in this position was one that was used for goat and sheep. It depends on the culture which of the two animal is represented. In Japan its the sheep so we'll go with that and there's an easy choice there: Mareep. The closest Pokemon I can think for a goat is maybe the Abra family though that's even a stretch and probably not what they were going for.
Monkey: Mankey, Primeape (though it doesn't have a tail) or Aipom, any of the three can work just fine here.
Rooster: And the last one which there's really no ideal choice. Starting I guess with the most obvious, in the first episode of the anime a Dodrio was seen crowing at the crack of dawn like a rooster does. But there are still other bird Pokemon. Roosters are well known for their comb and wattle and there's only one Pokemon that has a similar body structure: Fearow. But there's also a very strong case for Ho-Oh being the rooster, and not just cause its crest looks like a comb. Ho-Oh is based on the Chinese Phoenix, the Fenghuang. Though the Fenghuang is often presented as looking like a peacock, it's also called the "August Rooster" as it sometimes takes the place of the rooster on the, you guessed it, Chinese zodiac. FUN FACT: The Fenghuang used to be separated into males being called Feng and females being called Huang, however they eventually combined into a single female creature so they can be the counterpart to the male regarded Chinese dragon (so Gyarados is the opposite of Ho-Oh).
Dog: I would say Growlithe and Arcanine, but let's say that Arcanine was the actual representation for Tiger, what other Pokemon could be the dog? Well, Growlithe is still free but let's keep it one family per animal. Other dog Pokemon at this time were Snubbull, Granbull, Houndour, Houndoom, and Smeargle (we won't get into the other Legendary Beasts, Raikou being a sabertooth tiger seems to be one of the few things agreed about the group). Yeah, Gen II introduced 5 dog based Pokemon. If I were to pick one I'd pick either Houndour or Houndoom since they're still quadrupeds.
Pig: No mystery here, Piloswine. Though that makes me wonder, does that mean Swinub was designed later?
So, what do you think? A Gen II Chinese zodiac of Rattata, Tauros, Raikou, Azumarill, Gyarados, Ekans, Rapidash, Mareep, Aipom, Ho-Oh, Growlithe, and Piloswine looks fine to me!