When a dog is over threshold they may not be interested in treats. I often think about my first working dog Lucas who is the most treat motivated dog I have ever met. He has eaten himself to the vet on multiple occasions!
The moment he saw an armadillo near our campsite his motivation for treats had temporarily disappeared. I had never seen him turn down treats, except for medical reasons, so seeing him so excited about an armadillo and not even caring about the Kong peanut butter in a can was bewildering to me. The armadillo took his time leaving the campsite and it was quite obvious at that point that Lucas was not going to relax until the armadillo did. It took Lucas around 10 minutes before he started taking treats again since the encounter began.
If I knew then half of what I know now I'm sure I could have used that moment for teaching instead of waiting him out and trying to bribe him for his attention. This is especially true since Lucas has probably encountered maybe 4-5 live armadillos in his life.
What is an animal that your dog has only seen a few times and what did they do?