I don't think there should be any correlation between perception of pain and intelligence level, but from a cognitive psychological point of view, it might be useful to point out that perception of pain is associated with activity in the brainstem, in the pons, midbrain, medulla oblongata and thalamus-- structures common to any animal with a brain.

I would say the reason it would never have developed in plants is that they can't move, and thus can't react to stimulus. So perhaps perception of pain is positively correlated with ability to move, capping out at a certain degree of mobility.

Considering pet animals, including dogs, cats, and rodents, have been observed to develop Post-Traumatic Stress Syndrome in response to chronic physical abuse, there is no reason at all to assume they can't feel pain.

Lastly, I must point out that Descartes, whatever his contribution to mathematics, was a philosophical clod; his solipsist Cogito being the stuff of the thoughts of so many self-absorbed teens. There is abundant evidence that he thought animals did not feel pain because he believed it said so in the Bible.