Caballo vs. caballero
While the English word knight can describe both a medieval warrior and a chess piece, Spanish uses two distinct words: caballero and caballo. They are not interchangeable and refer to very different things.
Caballo
A1The word caballo primarily means horse. However, in the specific context of the game of chess, it is the correct term for the knight piece, which is shaped like a horse's head.
El caballo es la única pieza que puede saltar sobre otras en el ajedrez.
(The knight is the only piece that can jump over others in chess.)
Mi tío me regaló un hermoso caballo negro para mi cumpleaños.
(My uncle gave me a beautiful black horse for my birthday.)
La mejor jugada es mover el caballo a la casilla f3.
(The best move is to move the knight to the f3 square.)
Vimos un caballo salvaje corriendo por el campo.
(We saw a wild horse running through the field.)
El caballero montaba un caballo blanco.
(The knight (person) was riding a white horse.)
Caballero
B1The word caballero refers to a person. Historically, it means knight—a noble, mounted warrior from the medieval period. In modern, everyday Spanish, its most common meaning is gentleman. It is never used for the chess piece.
El caballero juró proteger el reino con su vida.
(The knight swore to protect the kingdom with his life.)
Gracias por su ayuda, es usted un verdadero caballero.
(Thank you for your help, you are a true gentleman.)
Los caballeros de la Mesa Redonda son figuras legendarias.
(The Knights of the Round Table are legendary figures.)
¿Me permite, caballero?
(Excuse me, sir/gentleman?)
Ese caballero sacrificó su caballo para ganar la partida de ajedrez.
(That gentleman sacrificed his knight (chess piece) to win the chess game.)
Summary
In short, use caballo when referring to a horse or the chess piece (knight). Use caballero when referring to a person, either a historical knight or a modern gentleman. A caballero (person) might move a caballo (chess piece).