Regarding St. Francis, he didn't so much have a doctrine of animals, and he didn't really encourage the rest of us to preach to the animals, or to the trees, though he paid particular attention to animals and trees as individual creations of God, not simply as background in some vague modern conception of nature. For information, here are a couple of passages from Chesterton on Francis:
"Everything would be in every sense a character. This is the quality in which, as a poet, he is the very opposite of a pantheist. He did not call nature his mother; he called a particular donkey his brother or a particular sparrow his sister. If he had called a pelican his aunt or an elephant his uncle, as he might possibly have done, he would still have meant that they were particular creatures assigned by their Creator to particular places; not mere expressions of the evolutionary energy of things."
And again, "But even apart from any miraculous powers,men of that magnetic sort, with that intense interest in animals, often have an extraordinary power over them. St. Francis's power was always exercised with this elaborate politeness. Much of it was doubtless a sort of symbolic joke, a pious pantomime intended to convey the vital distinction in his divine mission, that he not only loved but reverenced God in all his creatures."
So far as I have read, however, Francis made no claims that animals would be in heaven.
