Mewtwo appears as a raid boss for a limited duration. You can get Mewtwo by beating and successfully capturing a raid boss Mewtwo.
For how to beat Mewtwo effectively and best pokemon to beat Mewtwo, check the link below.
Mewtwo is weak to Bug, Ghost, and Dark types, while being resistant to Fighting and Psychic types.
Abra nests and spawn locations
Infestation
Goldeen nests and spawn locations