It's not just intelligence, it's awareness. A human would suss out that something was really wrong about the situation as they were being herded into an abbatoir. Other animals, including mammals, less so. I'd say a pig is more likely to sense a bad situation than a more stupid cow.
Then there is the experience we call pain: Almost all animals have neurons, not just mammals. Even earth worms and insects. So if your policy is not produce harm to anything with neurons you'd better not be crushing roaches. But pain isn't as simple as merely having neurons, it's about nociception and the experience of that. And pain is not merely nociception. Nociception is the brain receiving messages from pain receptors and we know various lower animals, including insects appear to receive and react to such signals. Pain is the experience created by the brain in response to nociception. Whether animals experience it as the same sort of pain humans do is unknown. Even in humans, the link between nociception and pain is not linear at all. You can have strong nociception and feel very little or visa versa. This is commonplace in observations of things like nerve pain from back injuries. It's possible insects suffer pain the same as us not not at all. We can't know for sure.
But some scientists believe that human-style pain requires a level of emotional sophistication to add meaning to the nociception. Therefore animals with higher emotional abilities (which is very much aligned with intelligence) might suffer more pain than less able ones. To whit, a pig may suffer more than a more stupid animal.
So you see my continuum argument is very much pertinent to the claim that all mammals suffer the same.