Petraverd wrote:...The only Talking Beasts we see choosing to turn for one reason or another are depicted as rather malicious sorts to begin with, and all seem to do it for the sake of power in one form or another - a distinctly human desire.
...but I think in most cases it wouldn't be enough for a Beast to justify defecting to the other side - not unless some more destructive sort of motive (greed, power, pride, etc.) was present.
This. The Beasts that we see turning from Aslan either were malicious to begin with or let a less than honorable motive take precedence. Some creatures are incapable of having this happen to them, due to their inherent virtue - such as we Unicorns. Others have the potential to go either way - as it is with most of the Talking Beasts. The capability is there - but when it does happen, it tends to go hand-in-hand with other things I've already mentioned, at least, as we see it in the Chronicles.