The game files themselves might not explain how exactly the creatures vary, but like you said that doesn't mean that the differences aren't there.
The game doesn't really seem completed for a lot of it, so even I'm going to take the game (and the booklet) saying that the creatures have different stats with a pinch of salt. That said though, there is still a noticeable difference between the creatures.
As for modding, as far as I'm aware no one's really found a way to mod creatures very much, those simple .txt files, skins and fur seem to be what it's limited to for the most part (though I think I read Daxter had found a way to make new creature meshes), so it's not exactly reliable to use one of those files to explain the workings of the creature. After all, it is record breaking AI
![Stick out tongue [:P]](/emoticons/emotion-4.gif)
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I'll admit, for the most part, the differences are subtle - but definitely there. I've very rarely managed to convince an ape to fight well in a creature/creature battle, but lions, tigers and wolves manage to a lot. As well as that, cows always seem to be more inclined to entertain the people.
Edit: I've just been having a go on the game again, with a few new save files, and undeniably the turtle has a much stronger defence than the rest. I suppose it could at least be possible to put some of the other factors down to chance, why you'd think that I don't know though, but the turtle definitely has stronger defence. The wolf also seems to be similar to the lion in speed, and the ape similar to the cow - both slower than the wolf and the lion. If you want, I could upload it as a youtube video
![Stick out tongue [:P]](/emoticons/emotion-4.gif)
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