We have been chasing the Legendary Beasts Raiku, Entei and Suicune for a long, long time in the Pokémon games. These Pokémon were among the first to have a special battle theme and guess what – they still have that theme today.
We aren't done chasing the three yet though, in fact, we can still chase them down in Pokémon Omega Ruby and Alpha Sapphire…only this time they aren't roaming around the region, the are tied to a single location called the Trackless Forest. To catch the Legendary Beasts you will the Eon Flute to Soar and you need to have captured Ho-oh or Lugia and one other legendary Pokémon. The 'other' legendary does not need to be any particular legendary Pokémon, it must simply be a legendary.
So, you've used our guide and secured youself a Ho-oh or Lugia, next up grab yourself any other legendary — it can be Reshiram, Zekrom, Cobalion, Tornadus (although using Latios or Latias might be best considering they can Soar)– any legendary you have!
Take your Ho-oh or Lugia and the other legendary Pokémon and put them both into your Pokémon party. Soar above Rustboror City and look towards the East, there will be a shining spot (a Mirage Spot) in the distance called the Trackless Forest. Land in the Trackless Forest and head over to the ring, interacting with the ring will take you to an encounter with Raiku, Entei or Suicune.
Before you head into the ring take a look at the time, depending on the time you will encounter a different one of the Legendary Beasts:
Raiku: ??:00 – ??:20
Entei: ??:20 – ??:40
Suicune: ??:40 – ??:59
** ?? stands for any hour, the time you want to pay attention to is the minute within the hour.
Once you have caught the available Pokémon, Soar to another area and wait for the next available Legendary Beast. Do not get to the Trackless Forest prior to the time switch.
Good luck and don't forget to bring HP restoring items!
Here's where you can find guides for catching legendary Pokémon and finding Mega Stones.
