This is Gastly. It evolves into haunter.

This is Haunter. It devolves into Gastly. It evolves into Gengar.

This is gengar and his gigantamax form. He devolves into Haunter. He doesn't evolve because he is final form of Gastly.

This is Farfetch'd. He evolves into sirfetch'd,but only if he gets three critical hits in battle.

This is Sirfetch'd. He devolves into Farfetch'd. He is final form of Farfetch'd.

note: I am going to stop saying the develotion.

This is Charmander. He evolves into charmeleon.

This is Charmeleon. He evolves into charizard.

This is Charizard. He is final form of Charmander.