So basically in The Magician’s Nephew we see the creation of Narnia and how the White Witch got there. And after creating all the animals Aslan picked out two from a handful of different species and gave them the ability to speak. He also made the small animals, like squirrels, bigger and the big animals, like elephants, smaller (but only the ones that could speak changed size). So the descendants of the speaking ones can speak and the non-speaking ones (which are literally called the dumb ones in the books) can’t speak. Some species can’t talk and aren’t intelligent, like the mice in The Lion, the Witch, and the Wardrobe. But bc the mice in that book chewed through the ropes that bound Aslan on the stone table they were eventually gifted with speech and intelligence which is why Reepicheep and his friends can talk by the time of Voyage of the Dawn Treader