You spend your life in search of the fifth horse.
As a child, you see only four.
As you grow, you proclaim the word "horse" itself must surely count as the fifth.
You squirm in your doubts. You seek to make sense of the illusion, conjuring a vision of a horse between the lines.
Were there ever four horses to begin with? Or has it merely been a single drawing and a great many assumptions?
Only after humbling yourself before the question do you begin to understand the truth: You, and only you, are the horse which you seek. The fifth horse had been inside you all along.