This page explains the idiomatic meaning of "Leopard cannot change its spot" after which examples follow.
This is an idiom taken from an animal's behavior. Leopard is an animal that also belongs to the family of cat with yellowish-brown fur with dark spot which can be located in some part of Africa and south Asia. Its female counterpart is known as leopardess.
Meaning of the idiom leopard cannot change its spot
This is indicating unchangeable habit of human being. It is used to qualify human being of a stable character that can never change for anything no matter what they pass through especially bad character.
Consider the following:
He still stole something from his neighbor's shop despite that he was just released after six months of imprisonment; indeed, his leopard cannot change its spot.