Suppose that animals were legally persons and every cow, dog, rat, etc., qualified for basic rights--essentially the right to life, liberty, and the pursuit of happiness. You would no longer be able to buy and sell animals, lock them up in zoos, kill them for food, experiment on them, etc. You can look at the arguments for giving them that status, and you may just find them convincing, but to start desiring that world, and really aiming for it, you've got to be able to imagine it. What all would be going on in a place like that? How would every day life be affected?
Enter, fiction. Unforutnately, I can't think of a novel that imagines what needs to be imagined--a world where animals are just "out there" in the wild, and not in our houses or grazing in pastures, or trapped in cages and stalls. A world where squirrels are reclassified as persons, so roadkill is not a laughing matter. In all the animal-elevating novels I can think of, the animals are not elevated in status as animals. They're Disneyfied --human on the inside, animals only on the outside. I can't think of a novel where genuine animals are treated as persons.
What we need is something along the lines of Margaret Atwood's novels Oryx and Crake and The Year of the Flood (parts I and II of a trilogy, and both excellent), which vividly imagine distortions to the world of animals wrought by environmental disaster and biotechnology. The normal species have gone extinct. All sorts of new life forms have been genetically engineered for fun and profit. There are rakunks (raccoon/skunks) and pigoons (pigs used to grow replacement organs for human beings). Mo'hairs have long tresses in multiple colors that can be harvested for human use. Apparently livestock have largely disappeared. There are headless chickens that yield chicken flesh without any suffering, and "real" burgers of very scary, mixed provenance, but mostly people eat a lot of soy pseudo-meat. If you wanted to think vividly about whether we should engineer new animal forms, it would be a very good idea to first read these books.
But what should we read before we decide whether to reclassify all animals as persons? We really do need something. We can philosophize about whether that's the right thing to do, but I really think the imagination has to play a role when we make up our minds about what to do, how to live, how to reconfigure the world. If we can't picture a future and tell coherent stories about it, then--hold on!--we shouldn't be aiming for it. So...
Wanted: a novel or movie with animals (real animals, not talking animals) elevated to the status of personhood. No need to go to extremes--those who want personhood for animals aren't saying they ought to be able to vote or enroll in public school or receive medicare. They're saying they should be legally protected from death, torture, and confinement. I'd like to read a good, vivid novel about a society in which that has come to pass. If you know of one, pray tell.