I tarded it a Ferro, from black 2, and then made an egg with it, and hatched the egg and in USUM. Can I use the egg Ferro in vgc?
Yes you can. The Ferrothorn was generated in Pokemon US/UM. You can check by the Pokemon's Origin Marker, which should be a Black Clover to be compatible for VGC.
Hope I Helped!
Players are normally required to use Pokémon that originated in the most recent video games. For instance, during the 2018 VGC season, Trainers can only use Pokémon that have the black clover symbol, indicating that they are native to Pokémon Ultra Sun, Pokémon Ultra Moon, Pokémon Sun, or Pokémon Moon.
Yeah. Anything hatched in Ultra Sun/Moon or Sun/Moon is legal.
VGC rules