Realistic animal stories are usually more about the humans involved than about the animals. This isn't always true (London's Call of the Wild is a notable exception), but anyone who's read The Yearling, James Herriot's 5-volume memoir, or most of Marguerite Henry's books knows that the main characters are people, and the animals are the primary concern of those characters.
That's not to suggest that the animals are incidental, just that what readers are supposed to take away aren't simply facts about horses or dogs, but what all good literature offers, insight and commentary on the human condition. The reason we like stories involving animals so much is that humans and their furry companions are able to forge unique bonds predicated on innocence and loyalty that rarely exist in strictly human relationships.
What sets realistic animal stories apart from fantasy animal stories is that the animals in them are real animals, not talking, clothes-wearing, pipe-smoking people in animal costumes. Dogs bark, horses gallop, cows give birth to calves (it happens a lot in Yorkshire, apparently), and the world is very much as we experience and know it to be.
Children are particularly drawn to these stories, not because they like animals more than adults do, but because their imaginations still recognize everything as fair game. Animal tales show the world from an often unfamiliar perspective, and the appeal is especially great for those whose own perspective is still largely unformed.
A lot of these make great read-alouds for really young kids, or good books for budding readers to practice on their own. Just because they're in the kids' fiction section doesn't mean they're only intended for children, either, and plenty of adults are still as much into animal novels as they were thirty years ago. Our selection is intentionally broad, and steadily growing—we may or may not be included in that last group.