You won't be able to catch it in the wild, so you can either restart the game with Charmander or trade it from someone who has one.
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Starter:Charmander-Suicune Squirtle-Raikou Bulbasaur-Entei
After beating the Elite 4 for the second time if you haven't already, one of the legendary beasts (Suicune, Entei, Raikou) will be released and be running around the world. Suicune if you started with Charmander, Entei if you started with Bulbasaur or Raikou if you started with Squirtle.
you can find them in Pokemon leaf green /fire red after you beat the elite 4 and the champion Charmander - Suicune Squirtle - Raikou Bulbasaur - Entei how do you find them?
The legendary dog which you face will alway be typed to be at an advantage against your starter. Therefore, if you started with Squirtle (which becomes Blastoise), then the dog you will face is Raikou. (Bulbasaur=Entei; Charmander=Suicune)
Suicune is not a possible starter Pokemon in emerald you must get a suicune from Pokemon firered or leafgreen.