From Bulbapedia, the community-driven Pokémon encyclopedia.
(Japanese: スイクン Suikun
) was a main character in Pokémon 4Ever
While on a ferry going to Ash and his friends' next destination, the group looks out into the woods and see Suicune. Not knowing what it is, they call Professor Oak and describe to him. Professor Oak says that it was in fact a Suicune and tells the group all about it. Ash then asks if Professor Oak ever saw a Suicune, and he replies yes. He then asks where Ash saw the Suicune. Ash tells the professor where he saw it and before Professor Oak could respond, the ferry owner calls for Ash. Ash, Misty, and Brock quickly say goodbye to the professor.
A while later, Suicune sees Vicious making Celebi destroy the forest and saves Ash and Sammy from falling off the monster that Celebi created. Ash and Sam, along with Suicune, eventually save Celebi from Vicious.
In order to heal Celebi, Ash and co. bring Celebi to the lake, but it doesn't work. Suicune then purifies the water, but the water still doesn't heal Celebi.
Before Sam and Celebi go back to the past, the group say good-bye to Suicune and it runs off.
For more information on this Pokémon's species, see Suicune.
It is never said in the movie whether this Suicune is the same Suicune that is frequently appears in the Pokemon anime.