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I'm thinking tiger, but he's more like a crossbreed.

Heh... I just thought of a fanfiction I read where Red, Cait Sith and Khimari (Dang, I bet I spelled that wrong...) Were working together on something and one person was all, "What the hell is with you people and all the talking cats?!" XD

Yeah... Please ignore the off-topicness. >_>

*throws in vote for Wolfishness*

...and that's only a 51/49 percent split. There are many characteristics that say both, but IMHO, his behavior and presence are more canine than feline. I'm relying heavily on his appearance in AC, to me it shows more canine resemblance. These are my supporting reasons:

From a Veterinary/scientific/Anatomical POV--

Examine the teeth - extremely pointy or not so pointy? cats tend to have a more "sharp knife" quality to them, while dogs tend to have a "table knife" appearance... both functional on raw meat, but the cat has the upper hand for being a true carnivore. Also, the number of teeth varies between cats and dogs as well. To this day, this is still one methodology (of many) used to determine ancestral relationships. I'm still hunting for a good screencap that would let me get "personal" for this one smile

Examine the snout - cats generally do not have an extended nasal region. If you were to look at various skulls, there is a definate difference between feline and canine. I would say that there is more length and "bridge" to his nose, irregardless of a wide-set nasal "pad" (erm, I forget the technical term *egads!*)

Examine the feet - although there really are no "good" screencaps of his foot pads, cats generally hold a more rounded shape to the overall "print" while dogs tend to be more oval or elongated. That and cats will keep their nails retracted (you see them only when they flex the foot) while dogs have theirs out normally. Again, I lack imagery that could be picked apart accordingly.

In some respects, he comes across like a Rhodesian Ridgeback on crack, with all that crazy hair XD

I've always thought he was a wolf with tiger features

i would say that but in advent children they make him seem like a dog/wolf

Nanaki, ie Red XIII is a beast, therefore he has no original root of species other than what he is. What genus Nanaki is from is not stated but what we can tell is that he has characteritics and apperance of a Wolf/Tiger. The wolf in him seems to be dominate more than anything else. Im pretty sure he's not a chimera but his species is never defined in the game.

some things are better left as a mystery