Monster Hunter Stories 3 has dozens of Monsties to collect, but the rarest monsters can only be obtained through mutations. They're by no means necessary to complete the story, but you'll have a much easier time with them in your party because of their boosted stats and improved genes.
Mutations are a part of Monster Hunter Stories 3's environment restoration system, which lets you release monsters to rejuvenate the local wild population. If you release a monster into the right environment with specific monsters present, you can end up with a powerful subspecies like Ivory Lagiacrus or Dreadqueen Rathian.
