Instructions
Complete the main game by defeating the Elite Four.
Walk through long grass. Entei, Suicune or Raikou will appear at random. There will be a powerful level 50 monster equipped with dangerous and tricky moves to prevent you from capturing it.
Use the Master Ball if you have it. Unlike other Pokeballs, the Master Ball has a 100 percent capture rate. If the Master Ball is unavailable, use Ultra Balls as an alternative.
Bring Pokemon that can inflict sleep or paralysis to keep the legendary dog from running away.
Check the Pokedex if the legendary dog runs away. The "Area" section of the guide will display its current location. Quickly go there for a chance to reunite with the dog.