There is a popular concept in animal rights philosophers, as advocated by Peter Singer (leading philosopher of modern animal rights movement), which is that if you were to mistreat animals based on their lower abilities in thinking, then it should be just as acceptable to mistreat people with disabilities and mental retardation or even babies. Other than discrimination against animals based on their cognitive abilities, what other reasons can one possibly give that justifies inhumane treatment towards animals?
Is the reason appearance and looks? Unjust discrimination of this kind is called speciesism, and if you admit to that you basically acknowledge to accepting racism against other humans of a different race or ethnicity. Because really, what is the difference between discriminating against animals and other human races based on outward appearance?
Or is your reason that animals are physiologically different than us? Sure, they are wired differently but did you know all mammals share the same basic nervous system as humans? This means not only that their body and organs function similarly, but also that they are fully capable of feeling pain and experiencing suffering. Animals are also sentient beings, having the ability to perceive and feel, as well as be conscious and aware of themselves and the world. Animals consciously want to be happy and to live happy lives, out of harm and danger's way. By choice, no animal wants to suffer and they can feel misery, unhappiness, and pain if say a human decides to kick it or lock it up eternally while causing open wounds on the animal's body.
That is the core of animal rights.
"The question is not, 'Can they reason?' nor, 'Can they talk?' but rather, 'Can they suffer?'" -Jeremy Bentham (first champion of animal rights)