Goh's Suicune travels the world purifying polluted water. A group of Pokémon Hunters were purposely polluting a lake to lure Suicune so they could catch it. They managed to tie it down when
Goh threw a Poké Ball at it and caught it as a way for it to get away from the Pokémon Hunters.
Goh was going to let it go, but Suicune decided to stay with him after it saw how it took care of it and tried to protect it.