Rights were devised by humans. How can they even be applicable to animals?

Just as the moral status of a human or animal is not determined by who caused the human or the animal to come into existence, the application of a moral concept is not determined by who devised it. If moral benefits went only to the devisers of moral concepts, then most of humankind would still be outside the moral community. Rights concepts as we currently understand them were actually devised as a way of protecting the interests of wealthy white male landowners; indeed, most moral concepts were historically devised by privileged males to benefit other privileged males. As time went on, we recognized that the principle of equal consideration required that we treat similar cases in a similar way and we subsequently extended rights (and other moral benefits) to other humans. In particular, the principle of equal consideration required that we regard as morally odious the ownership of some humans by other humans. If we are going to apply the principle of equal consideration to animals, then we must extend to animals the right not to be treated as a resource.

It is irrelevant whether animals devised rights or can even understand the concept of rights. We do not require that humans be potential devisers of rights or understand the concept of rights in order to be beneficiaries of rights. For example, a severely retarded human being might not have the ability to understand what a right is, but that does not mean that we should not accord her the protection of at least the basic right not to be treated as a resource of others.