Forge version: 14.22.0.2467
Twilight Forest version: 2.4.0.63
Link to crash log: (please use a paste site, do not attach the .txt or paste the log inline)

Steps to reproduce:

1. Have your world on any difficulty aside from Peaceful.
2. Spawn in/generate a Hydra, Phantom Knight, Twilight Wraith, or Naga.
3. The Hydra, Phantom Knight, and Twilight Wraith wont despawn.
4. The Naga will despawn but leave its model behind.

What I expected to happen:
The listed mobs to despawn upon setting the difficulty to Peaceful.

What happened instead:
They were still in the world set to Peaceful mode.

My assumption for the Hydra not despawning is because the Enderdragon does not despawn in Peaceful mode, and the Hydra has similar coding to the Enderdragon. Only a theory though