HeartGold & SoulSilver Roaming Johto
Wake the trio at the Burned Tower and two of them scatter for good. Raikou and Entei prowl the wild grass after that, each vanishing after a single turn of battle before you can corner it.
Roaming Johto is in Johto. In the story it comes after Route 39 and before Olivine City.
ROAMING JOHTO MAP
Pokemon Encounters
Entei and Raikou break loose after you disturb them at the Burned Tower and roam Johto from then on, both at level 40. Tracking the pair across routes takes patience and a Pokemon that can pin them in place. Suicune sits this hunt out entirely, pinned instead as a fixed level 40 catch on Route 25 over in Kanto.
Entei
Roaming Johto Guide
After fleeing the Burned Tower, Raikou and Entei become roamers at Level 40 that jump to a different route every time you change areas. On the first turn of a battle they try to flee, so a paralyzing or trapping move helps pin them in place. Once you have seen a roamer, the Pokegear Map shows its current location.