According to the game, and bulbapedia, Suicune is gender less. However, in Pokemon Mystery Dungeon, it's a boy, yet most internet artists depict it as a girl. Hope this helps
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after you have defeated all 16 gyms go to were you found misty and his boy and suicune will be there.
boy
His Snivy is a girl. And Pikachu is a boy.
suicune costume
a boy.