Ok I forgot that my dogs can talk. My bad.
A Bulldog I had was found one morning with a swollen leg. It was not like that the night before so I rushed her to the vet. Cancer. Tumor had cut off pathways overnight and her leg was swelling because the fluids could not get out. A bunch of test later they found the tumor was huge and she was not going to make it. So from good to bad with no signs hardly at all.
My first Pitbull was perfectly fine one morning when I left for work. A few hours later my wife got home and tried to take him on a walk and he didn't want to go. I left work early and took him to the vet. Cancer. Tumor this time had ruptured and he was bleeding internally. Best case was $14K in care and the treatment and surgeries alone could kill him. So again from good to bad with no signs hardly at all. He was running around and acting as normal as could be before I left only to be down a few hours later with no obvious problem.
You want to know how many times I went to get eggs and found a chicken dead? It was numerous. Fine one day and dead the next. Old age? Cancer? Something else? I don't know because it didn't tell me.
I have had rabbits do the same way.
In general animals do not always do a great job at showing illness or issues.
So I will compare animals however I please from my experience with them.