In the IMF Recon Module, Rulen mentions cats.
Cats are only on our planet, right?
Or are there just cats scattered all over the universe, on all sorts of different planets?
Or just on Earth and the Spiral Knight's original planet?
Knights are from Isora, not Earth.
What if that's just their name for it...?
"What's this? More wolves hungry for the blood of Almire? Our great kingdom shall never fall to the likes of beasts!"
-Lord Vanaduke
There can be cats and wolves and other animals on Cradle or from any planet that the Knights are from (I thought they were from Isora?). Just because there's savage monsters on Cradle that happen to take the form of common animals doesn't mean those common animals don't exist altogether on Cradle or Isora or wherever.
"What if that's just their name for it...?"
Sorry, but this argument holds no basis. I could very well say that Cradle is Earth and you can not reliably prove me wrong because you don't know that it's not Earth. Therefore any planet or, heck, even small biomes could be Earth (we don't know our relative size compared to Earth, we can just make logical guesses). A wolver's eye could very well be Earth, assuming a wolver's eye is round.
EDIT: You're also assuming that he was saying that there were cats on Isora, and that they didn't meet their first cats when they came to Cradle, which is another possible argument.