What is crest on a horse?

The "crest" on a horse refers to the mane, which is the long, flowing hair that runs down the top of the horse's neck.

Sometimes the term "crest" is also used to describe the topline of the horse, which refers to the entire line of the horse's back from the withers (the bony prominence between the horse's shoulder blades) to the croup (the rump). This use is less common, though.

When someone refers to a horse's "crest," it is usually safe to assume they are talking about the mane.