The whole CAN_LEARN/SLOW_LEARNER thing needs an overhaul, or at least a rethink IMO.
I've always seen trolls as being basically like monkeys - they can be taught some skills, like how to fight, perform tricks, and if they have hands they can learn to perform basic labors, but can't complain, plan vengeance on their oppressors as a group, or petition for equal rights, and therefore they are never actually given the rights of sapient beings. While animalpeople are just...people who happen to have the heads of animals, and haven't formed a civilization for whatever reason, but would be biologically capable of doing so.
Except that logically this distinction really falls more under the ability to speak - the ability to create a social contract - not the ability to learn skills. Pretty much all animals with brains have the ability to learn skills, to a greater or lesser degree, but DF ignores this and uses the CAN_LEARN token to determine whether the species should be regarded as sapient as far as cannibalism and killing are concerned.
This is probably because there are humanoid creatures in DF who cannot speak, like plump helmet men, kobolds, and trolls, and the idea of dwarves eating humanoid, thinking beings because said beings aren't capable of organizing and fighting back rubs Toady the wrong way.
But if so, I think there should be a separate token to handle this, like CONSIDERED_SAPIENT, because the idea that a creature is either "fully sapient and capable of creating a civilization" or "mindless automaton incapable of learning" is kind of silly, from both a realism and a gameplay perspective. (Wouldn't it be neat if your pets could learn combat skills, and an old, experienced war dog could fight better than a just-trained one?)