It’s common in most animal groups to be suspicious of other species. It was more prevalent in the original movie, as you had to be VERY careful outside of the Great Valley. Now however it seems that Mr. Threehorn is the most racist of all the Great Valley dwellers. He’s a natural protector, and would likely do VERY well in the Mysterious Beyond. It’s probably difficult for him to transition, as he’s likely spent most of his life outside the valley.