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

The inability of non-human animals to understand the concept of rights or enter into a moral contract with humans is irrelevant to their moral status.

There are many instances of humans who for various reasons may not understand the concept of rights or enter into a moral contract whether they are children or a human that has a severe intellectual disability. We still extend the same rights to these individuals as other humans, if not more rights in an effort to protect their interests. Animals similarly would fall into the category of being unable to assent to a moral contract but nonetheless possessing interests worthy of being protected.

Additionally if moral benefits were only provided to those who devised moral concepts the majority of humans would lie outside the moral community. Originally, rights concepts were devised as a way of protecting the interests of wealthy white male landowners; indeed most moral concepts were developed by privileged males to benefit other privileged males. We have since extended these concepts to include other humans as we recognised that the principle of equal consideration requires that we treat similar cases in a similar way. In particular, the principle of equal consideration required that we regard as morally objectionable the ownership of some humans by other humans. If we are going to apply the principle of equal consideration to animals, then we must also extend to animals the right not to be treated as a resource.