Strangers don't tend to be designed with a theme already in mind, and specific intent is often counterproductive. The characteristics of the strangers are instead discovered through meditation, free-writing, and in response to the sensory world. The importance of the editing stage cannot be understated, as well; a lot of the time I'd end up with more details than I really knew what to do with, and so it's in paring things down, that you can start to refine the stranger's overall nature as a whole.

It's also worth noting that you don't want them to be too "on-point" or "neat" – the discrepancies and small contrasts and irregularities are a big part of what gives a strain character. It helps to look at them as having details that surprise, but don't disrupt.

With that said, strangers which parallel or incorporate animal traits should be carefully weighed, and avoided if not with intent. Strangers can only reflect the way people view and treat animals; they cannot reflect animals themselves. They are more like the word "cat" than they are like a cat.