2 monsters I can think of off the top of my head that started out pretty tough but are now 'not so much' are the Will 'O the Wisp ans the standard Mummy.
Will 'O the Wisp was a 9d8 HD, AC-8 creature that was immune to all spells except Maze and Magic Missile. Now its 9d4 HD, still has a lot of immunities but is only a CR2 monster for 5e.
The Mummy needed a +3 weapon to do full damage. Now it has resistance to non magical weapons and is considered CR3 for 5e.
Will 'O the Wisp was a 9d8 HD, AC-8 creature that was immune to all spells except Maze and Magic Missile. Now its 9d4 HD, still has a lot of immunities but is only a CR2 monster for 5e.
The Mummy needed a +3 weapon to do full damage. Now it has resistance to non magical weapons and is considered CR3 for 5e.