You Can Catch Only 1 Legendary Dog Per Game.Which Dog It Is Depends O What Starter You Chose.The Dogs Type Is Super Effective Against Ypur Starters Type.The Dog Changes His Catch Area Whenever You Go Throught Something (Example : If You Enter A House,Get In Another Route,Bleh) The Dog Is Always Level 50.And , DONT FORGET , The Dog ALWAYS Flees After First Turn.
Thats All I Can Say.
(I Caught My Suicune While Searching For A Sandsrew xD)
Hope This Helps!