Earliest Pseudo legendary is Beldum. You can get it after you defeat Ciel and returns to restored Reborn.
Other pseudo legendary Pokemons:
- Dratini: Requires Rock Climb
- Larvitar: Requires Rock Climb
- Bagon: Requires Rock Climb
- Gabite: After Adrienn
- Deino: Requires Rock Climb
- Goomy: After Adrienn
- Jangmo-o: Requires Rock Climb