I will admit it right here — I talk to my cat. The day he says anything more than “meow” or “purrrrrrr” in response, I will get myself some help. But for now, I think I’m safe in the assumption that, although he communicates quiet well — scratch behind my ears, I’m lonely, I’m in pain, I want to sit on the newspaper, where’s the cat food, tickle me under the chin, etc. — he does not speak. His plaintive yowl when he was having digestive problems was VERY articulate, but it did not include any of the words a human would use to describe his discomfort.
In fiction, talking animals are most often surrogates for human beings. This applies to both children’s literature and books aimed at adult audiences, In fantasy, magical or otherwise invented worlds in which animals speak, they may speak to humans or replace them entirely as protagonists. In mysteries, I’m not overly fond of talking critters, as some of this devolves into parody or is simply too cute for me.
On the other hand, the animal aspects of a monstrous creature — like a werewolf or a dragon — should inform the creature’s spoken words as much as claws and teeth would shape the sounds of those words. Just thinking out loud here — what kind of vocabulary would suit a dragon?
Umm… something to ponder, or Roaaaaaaaaaah, something to poaaaaaaaaaander…