It's all old hat, but it's very effective. You can explain it in a single sentence, because everyone knows what a giant slug looks like, and everyone knows what a cow does. And so without being threatening or confusing, the idea of spotted slugs munching on bales of hay in some bucolic oasis reinforces how alien the setting is.
You just don't want to overuse it, or else it becomes comical (Gamma World, the one normal bear in the entire world of the Last Airbender), which might be what you're going for (Gamma World is great). Otherwise, stuffing one animal into another animal's skin works great.
So here's some otters in wolf suits.
