When staring with Bulbasaur (for Entei) or Squirtle (for Raikou) in Leaf Green or Fire Red, you will have a chance to catch their respective Legendary Beast. Both Entei and Raikou have the tendency to use Roar. If you encounter the legendary beast and they use roar, it is the equivalent to fainting without the experience. The Legendary will not appear in the same game again. To fix this, you must reset and try again. All data after the last save point will be lost. There is currently only one move to prevent this from happening. It is Ingrain. Ingrain will null the roar and you will be able continue as normal until it runs away (if it does). This does not affect Suicune as it does not have the move Roar.