I come at portraits from the position of shooting show formals where you don't really get to see any personality of either the dogs or the handlers. They are soooo boooooring
. But, that's the genre and that's what you have to shoot for.
With portraits, we really work to get some reaction from our subjects -- whether pets or people actually. So, using props, food, squeekys, toys, etc. anything that will get a reaction from the subject is fair game. Is it "cheating" to coax expression out of the animals? I don't think so. Their reactions are true to themselves. For example, a dog with very little play personality, won't cock it's head for a squeeky. A cat with the lights on, will give you the type of pose you see in so many of Preston's images.
Julie points out that the image should produce some type of emotional reaction. The mouse made me literally go "wow"; your shots of Yoshi, the stray kitten, made me say "great kitten". Those images conveyed something that made me react to them. I believe that's the objective.
Now, I'll admit that you can't always get what you want (was that a Rolling Stones song?). Some animals won't react in a particular situation; some will only react to their owners; some won't react at all. But, I think beyond getting it right technically, that's the challenge in pet photography. Getting them to show us themselves. Isn't that the same as with people portraits? Why should animals be any different?