The official Pokémon website has announced that for a limited time until November 4th, 2020, players of Pokémon Masters EX have a chance to obtain a Shiny Lapras or Shiny Mankey by hatching Eggs.

Hatch Shiny Lapras or Shiny Mankey in a Pokémon Masters EX Ice- and Fighting-Type Egg Event

Take a crack at getting awesome Eggs for a limited time.

Through November 4, slide into Pokémon Masters EX for an exciting Ice- and Fighting-Type Egg Event. During this event, look to train up your Ice- and Fighting-type sync pairs—they'll come in handy when facing the toughest challengers.

After the battles are over, Pasio's Professor Bellis will reward victorious Trainers with Eggs. Trainers can improve their Egg rewards by adding the player character to their team and by forming a team with sync pairs who have supereffective sync moves. Some of the Eggs could contain Lapras or Mankey, and Trainers may have a chance of obtaining a Lapras or Mankey that's Shiny!

Happy battling!

Source: https://www.pokemon.com/uk/pokemon-news/hatch-shiny-lapras-or-shiny-mankey-in-a-pokemon-masters-ex-ice-and-fighting-type-egg-event/