The difference is that when an animal suffers, they retain memories. One only need to consider something like this story I heard about a dog on Reddit, who was scared of obese people and men. Turns out, the dog was severely abused by a 500-lb man who was its former owner.
I then realized that animals can suffer, and either I’m confounding Buddhist and Western psychological definitions of suffering, or suffering doesn’t require consciousness of it occurring for it to be suffering.
I hope that makes sense!