They're unique in the sense that they're not the only living species on the planet. :P That's the easy answer, really I'd say the only thing that makes them truly unique is that they're the only other Pokemon aside from Volbeat and Illumise to have a true male and female split. The rest of the Pokemon don't really count in that they look exactly the same (smaller whiskers/tail/Wobbuffet lips or not). You'll never get a Nidoqueen that looks like a Nidoking, and vice versa, same for all the others. Also, they have the second largest family. It used to be the largest in the 1st generation, and then tied in the 2nd. They're going to keep capitalizing on Eevee's popularity, though, so who knows how many evolutions it'll get.
Even Ralts isn't too special considering you can still get a Male Gardevoir. Same goes for Snorunt.
Statwise and moveset though, yeah. Nothing.
Also, I do agree with the idea of liking strange creatures over the more generic (or actual) animals. Personally I prefer video games like this, it's why I'd play Kirby over somthing like Donkey Kong Country (gameplay values aside). I get bored drawing common animals as well. There's no denying the Nidos are either very animal like (Nidoran M/F, Nidorino sorta) or beast like. In fact Nidorino and Nidoking are very much kaiju-esque to the point of possible intention considering that they look to be inspired by baragon. Nidorino
is one of the earliest Capsule Monsters, afterall (aha, THERE'S another unique quality. Nidorino's one of the grandfathers of the series).
I think for me, though, the reason I like Nidos (and to that degree, Vulpix) over plain old rabbits and rhinos (and foxes) is because they're
just different enough to not be a garden variety animal, but not so out there that you can't really make heads or tails of it (and in Vulpix's case, it's the cutest fox character I've ever seen. There may be others I haven't been acquianted with, but the only other fox character I like is
classic Tails). Nidos are essentially chimeras, because they have a lot of different characterstics of lizards/dragons and mammals. That also keeps them from being a played straight rabbit like Buneary (even though Nidoran is ironically
more played straight anatomically). I've actually talked about this for pages before, I'll admit I don't really feel like going over it again, so I'm gonna stop cuz the lazies are getting the better of me.
So anyway, what you like are the weird, unidentifiable creatures the most (and Sandshrew). I can't exactly help you there. I still think overall, though, personality is what makes the difference in appeal. That applies to all living things. I'm not much of a dog or cat person, but even I'll admit I like the videos or picture of the more playful and curious ones. I'm sure even the most admant anti-cat/dog person has seen something or had a personal experience with a particular cat/dog and said "okay, THAT was funny". But the ones that are ordinary or lazy won't win anyone over unless they have an affinity for the animal to begin with. Same could be said about a very weird looking animal at a zoo. It's so cool and unique looking, but if all it does is sleep, most people are gonna be like "ok... what else does it do?" So, again, it's appeal is limited to those who have an affinity for it, or those who just like the bizarre for the sake of being bizarre.
Same can even be said for humans. How often does anybody really ever take notice of everyone in a crowd? Unless there's a spark of attraction ("Oh, s/he looks cute") or some unfortunate deformity (or you're on a bus bored out of your mind or are drawing people for practice :P), you'll just scan past and ignore everyone. The only ones who'll stand out are the ones you got to know for their personality (or you had to be in the same vicinity with them for a long time, or have an interesting face to draw 9v9).
That's really all I got. I can't really see Pokemon as just creatures at face value, because if I do, I'll ignore them all, Nidos included. It's a point of view I understand, but just can't have myself, even if I forced it. Koffing's extraordinary, no contesting that, but what has the most basic one done to make me care? I've been weaned on cartoons, comics and graphic novels like Bone all my life, so I sort of need a draw. That may be the reason why I see things so differently. Without empathy, there's little more than shallow interest from my perspective.
That's probably why I love these.
(Why didn't I pack these comics with me? ;_;)
Edit: Holy friggin crap the filters are
not kind to this dude. Have some tiny urls to get around it. Also, careful with that last one. It's a sight gag, but yeah.