英文諺語 A leopard cannot change his spots
A leopard cannot change his spots 江山易改，本性難移
A person’s character, especially if it is bad, will not change, even if they pretend it has.
From the Bible, Jeremiah 13:23 (King James Version):
“Can the Ethiopian change his skin, or the leopard his spots? then may ye also do good, that are accustomed to do evil.”