Well... this is a matter of some debate currently. The philosophically ideal solution is to create new topics for every fictional rendition (or interpretation or opinion...) of anything, but this makes for complex schemas and tends to make querying and browsing more difficult, and I'm not sure what it really gives us that's useful in most cases.
I'd be inclined to say that Dogbert's species is plain old Dog, even though real dogs can't talk, because, as it is, we aren't actually asserting that "dogs can talk" when we make that link. Real dogs also don't save children from wells out of their own moral imperative every day of the week, but it seems very pedantic to create a new topic just for "Canines in the Lassie Universe."
The only way I can think of for deciding how to draw these lines is to consider what is the most useful and practical. We also have to consider what pattern of data people will actually input, and I expect that most users wouldn't think to create a new topic called Dog, link it to the Dilbert Universe and assert that it's "based on" the Dog of reality, but rather link directly to Dog because they intuitively know that we never talk about the fictional species to which Dogbert would belong in a philosophically ideal world -- he's one of a kind, and any interesting things about him will be connected to the Dogbert topic itself. Wookies, on the other hand, though also "based on" the canine, really are thought of as a distinct fictional species -- there are many instances of them, and Star Wars geeks would happily discuss all of the species' various fictional social and biological features.
In our framework, it's trivial to look for "animals that are also character species" to see exactly where these cases appear, and you could then (programmatically, if needed), cleanly split all of these cases into two. In other words, I don't see any pollution of either the animal or the character species namespaces with this type of convergence right now, and if we ever want to be a little stricter about it, we can easily migrate there. For now, I don't see how a topic just to represent how Dogbert is dissimilar to real-world dogs would help anybody answer any questions, but I can imagine how it would make things a little more confusing.
Anyway, everything is in flux, and that's sort of the point. We aren't claiming to have an end-all solution to the philosophy of knowledge, we're providing a powerful and flexible framework that can gracefully evolve to accommodate all sorts of different ways of looking at the world. These models will always be imperfect; they wouldn't be models if they didn't carry assumptions. As Freebase evolves, we'll all have a better idea of what modeling patterns work best for solving the problems the community cares about.