Q: When I write stories that include horses, is it grammatically correct for me to say "he" or "she" when I write about a horse? Also when referring to a horse in context, can I write "who" and "whom"; e.g., "Whom shall I ride today?" —Hans C.
A: It's not often we get grammar questions about animals—it's even less often that we get one with two different answers. But that's what we have here.
An animal is referred as “it” unless the relationship is personal (like a pet that has a name). Then it’s OK to use “he” or “she" when referring to the animal. This also applies to using “who” and “whom." If the animal has a personal relationship with the person, then use "who" or "whom." Otherwise you must exclusively use “which” or “that.” Here's an example that incorporates both of these rules:
Personal: My horse, whom I call Steve, is my best friend. He comforts me when I ride him.
Generic: The stray dog, which I saw chasing its own tail, was shedding hair.
The "personal" rule also holds true if you're writing a kids book and the animals can talk—as you’re giving them human traits and making them characters your readers can get to know. Even if the animals don't have specific names, they are given personalities and this is enough to make them personal.