A horse that fits together well and has equal sized fore and hindquarters is said to be...

A horse that fits together well and has equal-sized fore and hindquarters is described as balanced. Balance in a horse refers to the harmonious proportion of its body parts relative to each other, enabling the horse to maintain stability and perform well in various activities. A balanced horse will have an even distribution of weight and a well-structured frame, contributing to its overall athletic performance and movement efficiency.

Proportioned refers to the relationship of the different parts of the horse to each other, but it does not singularly imply that those parts are in harmonious balance. Symmetrical describes the equidistant nature of both sides of the horse but does not specifically address the balance of fore and hindquarters. Compact indicates that the horse has a shorter structure, which may or may not involve balance, as it primarily refers to the physical size and build. Thus, balance is the most accurate term to describe a horse with equal-sized fore and hindquarters fitting together well.