Idea 1:
My first idea was spawned by Albertus Magnus. Albertus states that animals all act in the same general ways (because of the "natural powers" which dictate the passions and actions of a species)1. This general way of thinking about the actions of animals (that they have predictable and common habits and actions) should carry over into literature. For instance, if there are two foxes in a fable, they should have similar personality characteristics and perform similar actions.
However, we come into further complexity when we talk about talking animals. Speech, at least in the modern sense, conveys intelligence and memory (and probably free will). We choose what to say, formulate it based on our intelligence, and often reference memories of prior events or knowledge and extrapolate them to the current situation. Although medieval scholars indicate animals can produce sounds to communicate certain things (Albertus, for instance, states that birds make certain sounds when searching for a mate), I doubt any of them would have called this "speech". Similarly, although Albertus talks about training animals, he does not express any belief that this learning indicates an understanding of speech similar to that of humans. According to Aristotle's tripartite model of the soul (which Albertus uses), although animals have sensory abilities and some cognitive ones (estimation, imagination, memory of a sort), animals do not have higher reasoning abilities like intelligence and free will 2.