One of the more interesting chapters of Gary Steiner's Anthropocentrism and Its Discontents discussed Descartes view of duties to animals. Steiner essentially defends the orthodox (and obviously correct) view that Descartes viewed animals as mere biological automata, without subjective experience of their own. Some recent writers have dissented from this orthodox reading, but Steiner argues pretty persuasively that they are mistaken. Descartes in fact denied that animals were meaningfully sentient, and thus that we could have any obligations towards them.
As I've stated, the landscape of historical positions on animal ethics is relatively flat and unvaried. Kant, like Descartes, believed we have no direct duties towards animals. He argued only that we ought not treat them maliciously, as that would cause us to become callous toward violence, and make us more likely to wrong rational agents (the true subjects of worth.) Most people agree that this is quite a weaker source of obligation than they find plausible. But, in fact, it's worse than that, because the actual implication of Kant's view is that if you were an ideally rational agent, you'd be permitted to treat animals however you like. This is because as an ideally rational agent, there would be no chance of your maltreatment of animals carrying over to your treatment of humans.
In his chapter on Descartes, Steiner often suggests that Descartes' view is now considered quite repugnant. And so it is. However, it occurs to me that there's a certain sense in which it is far better than Kant's view, because Kant didn't deny that animals were sentient. In fact, he argues that they do have their own subjective experience of the world. He just denies that they have any moral status worth respecting.
In contrast, however, Descartes felt the need to deny the sentience of animals in order to justify their lacking moral status. Descartes knew that if animals had no subjective experience of their own, then they could not be wronged or harmed, and we could have no obligations to them. Such an ethical view is correct. What he got wrong was a metaphysical claim; it's now considered relatively indefensible to deny that (many) animals are sentient. But having wrong metaphysical beliefs seems less condemnable than having the correct metaphysical beliefs (as Kant did about animal sentience) but failing to see the ethical implications of such a view.
It is not clear what Descartes would have thought ethically about animals had he realized they were sentient. But Kant certainly did think they were sentient, and constructed an entire ethical system that nevertheless saw them as worthless. This, for me, is far more repugnant than Descartes' implausible metaphysical beliefs.