In my story, my narrator has only recently met other humans and the biggest creature she has ever interacted with are dogs. In the survivors' camp, she encounters their horses and is pretty freaked out, but I don't really know how to describe them in the way that someone would if they'd never seen them before. Help?
She would focus on both the familiar and the parts that are out of place.
* It's an animal (mammal) (4 legs, hairy)
* It works in a helper capacity to humans
* Its size! This is probably her first impression.
* It can be ridden. If this is part of her first view, it will likely be paired with size for her first reaction.
* Long face, neck, mane, and tail unlike any other she's seen.
* Those feet! Hooves not paws.
She might at first use words like "beast" and "monster." Some of the familiar vs the new will be simultaneous and in other cases she'll go back and forth while she gets her bearings. Not all new animals elicit this type of reaction, but one that is 10 times the size of any she's ever seen (and/or being ridden) surely will.
You could look at how children perceive animals that they do not yet know. What they do is subsume animals that look alike into the same category. At first every animal is called a "dog" (if a dog is the first animal a child knows). Then different classes of animals are differentiated, as the parents give names to the different species, and eventually the child will learn to recognize a new animal as different from the ones he or she knows and not give it a name that doesn't apply, even though she doesn't yet know the correct one.
So, if your narrator is mentally immature or doesn't much care about animals she might first think the horse is a big kind of dog and call it "dog". If she is of adult intelligence, she might see the similarity but notice the differences (mane, different tail, different teeth, different feet, etc.) and think of it as "a dog-like animal" or, if she is more scientifically inclined, as "the big quadruped". She might even be able to tell the predator from the herbivore, because a similar difference might exist in her home, and she might think of the new animal along these lines, e.g. as "the big herbivore" or, if she encounters the horse in its capacity as a mount, as "the mount".
Much of what she will call the horse will depend on the circumstances of the encounter (being ridden or on a pasture, from afar or suddenly upon her) as well as her personality and what aspect of it she will focus on most (the smell, the neighing, the size, the shape, the color, etc.). So your answer will very much depend on the story and the characterization you have chosen, and you need to derive at least part of your answer from that.
Horses are very much like ramped up dogs; the long face, the legs. They have hooves instead of paws. You have to add on manes and hairy tails. But if somebody has seen large dogs, like a Labrador Retriever or Greyhound, the horse is very similar.
I'd look through breeds of dogs and see which one looks MOST like a horse, and describe it that way. Just imagine that dog five and a half feet tall at the shoulder.