As above, you can find an animal with essentially any amount of brain, from close to a single functioning cell, all the way up to whales. That isn’t a useful definition.
Personally I think it is a Red Setter. Closely followed by human beings. I would define stupid (here as a coloquial use - not the OED where stupid derives from stupify) as an animal that actively does things that would qualify for a Darwin Award.
I have known a red setter, upon being taken to the beach, to jump in the sea and start swimming. Straight out. And keep swimming. And swimming. The owners found a boat and rowed out and hauling him into the boat and took him back. The next stop would have been Antarctica.
Actively doing things that clearly will put their survival in jeopardy is not something most animals will do. Not unless there is a deep drive - like protecting young. But humans, we do it all the time. That is what I call stupid.