A Leopard Can't Change His Spots

You cannot change who you are/change something’s nature

From the Bible, Jeremiah 13:23: "Can the Ethiopian change his skin, or the leopard his spots? then may ye also do good, that are accustomed to do evil."

Ex: I doubt very much that marriage will change Chris for the better. A leopard can’t change its spots.

Spanish: aunque el mono se viste de seda, mono se queda German: Man kann nicht über seinen eigenen Schatten springen.