What is the name for a small indentation typically located along the side of a horse's neck?

The small indentation typically located along the side of a horse's neck is referred to as a "prophet's thumb." This term describes the distinct marking or indentation that can occur where the neck meets the shoulder or alongside the neck. It is often recognized for the way it resembles a slight thumbprint or indentation on the surface of the skin, and it can sometimes indicate a certain conformation or condition in horses.

In contrast, the chin groove is a different anatomical feature located beneath the jaw of the horse, while a whorl is a tuft of hair that grows in the opposite direction of the surrounding coat, usually found on the forehead or neck. The dewlap refers to a fold of skin or loose skin beneath the jaw or throat area, which is not relevant to the neck's side indentation.