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"If you're so sure what it ain't, how 'bout tellin' us what it am?" Both Nidoran are probably a cross of a rabbit for prominent features, a hamster, and a hedgehog, or at the least has the quills of one, just based on the facts and obvious design qualities alone. Keep in mind, some animals may not be necessary. A hamster might be overkill since it's only used for the whisking, and that could be the fault of Nidoran's designers not knowing any better. It might even be a rabbit only.
Some people may say that Nidoran looks like a guinea pig. Overall because of the stoutness and round faces. This is sensible, but keep in mind Pokemon anatomy on the whole is super deformed from the start (so stoutness is a given), plus Nidoran male's face is much more tapered like a rabbit. Some people may refer to Nidoran as a pacarana, mostly because of the "ran".
Here, you see clawed paws, something that the rabbit doesn't have. However, what you don't see in this picture is that their bodies are also rather long, and so are their arms and legs. It's not very proportionate to Nidoran, even if you consider the super deformed design. Further, the ears are too small and they have long tails. Nidoran don't even have short tails. A pacarana is actually closer to a beaver.
If you consider all breeds of rabbit, you'll find some that look like Nidorans. Remember, not every rabbit is a lop or cotton tail. Some have small ears and a stout body, enough to accomdate even the female Nidoran's design well. Checking with Japanese interpretations for consistancy(as there is more knowledge about everything that originated from there anyway), Nidoran tend to be associated with rabbits categorically (usagi), as well as artistically. Sometimes they're grouped with other rabbit Pokemon in pictures with Lopunny and Azumarill. If you also take Japanese information into account, the "ran" in pacarana is most likely coincedental, much like the "pika" in Pikachu. "Nido" is a romanization "Needle", but I have not read any connections for "ran", it's possibly a meaningless suffix (or not, considering almost every other Pokemon is a combination of two or more literal nouns or adjectives, unlike localized names that are just punny because western localization gets off on that sort of thing.
So it all sounds good, right? Even when checking the Japanese consensus, Rabbit seem to stand out as the core animal for Nidoran, and with specific details we've at least deduced as much. Granted, just looking at the Male Nidoran's head complete with the cartoon buck teeth should have been enough, but you know some people.