Flutter Mane is weak to Ghost- and Steel-type moves because it is a Ghost/Fairy Pokémon. The official Pokédex lists those as its weaknesses.

Quick answer

  • Weak to: Ghost, Steel
  • Immune to: Normal, Fighting
  • Best caution: its low physical Defense means strong physical hits can hurt it more than its stats suggest

Easy example

A Steel-type attack like a strong Steel move can hit Flutter Mane super effectively, while a Ghost-type attack also exploits its typing.

If you want, I can also give you:

  • its resistances
  • the best counters
  • or a full type chart for Flutter Mane