If you expanded the prompt to describe the squirrel in redundantly verbose detail it would probably pick it up just fine. The more synonymous descriptors you can work in, and the more naturally you can phrase it, the better it will work.

But IMHO the interesting thing about controlnet is being able to use pre-rendered production art as a basis and allowing SD to respect the original proportions/model/etc. For rough sketches I prefer img2img without controlnet as it gives the algorithm leeway to fix, reinterpret, or "be inspired by" my input image without being too attached to it (since it's full of imperfections anyway).