Johto Gym

After you defeat the Gym Leaders, Elite 4, and Champion, you will unlock the following legendaries:
 * Raikou
 * Entei
 * Suicine
 * Lugia
 * Ho-oh
 * Celebi

Gym Leaders
Falkner- 

Violet Gym/Zephyr Badge

Pokemon:

Pidgey lvl. 7 (Gust, Tackle, Quick Attack, Twister)

Pidgeotto lvl. 9 (Gust, Quick Attack, Twister, Wing Attack)

Bugsy- 

Azalea Gym/Hive Badge

Pokemon:

Metapod lvl. 14 (Headbutt, Tackle)

Kakuna lvl. 14 (Headbutt, Tackle)

Scyther lvl. 16 (Wing Attack, Tackle, Quick Attack, Pursuit)

Whitney- 

Goldenrod Gym/Plain Badge

Pokemon:

Clefairy lvl. 18 (Pound, Doubleslap, Wake-Up Slap, Meteor Mash)

Miltank lvl. 20 (Stomp, Tackle, Body Slam, Rollout) Morty-

Fog Badge

Pokemon:

Gastly Lvl. 21

Haunter Lvl. 21

Gengar Lvl. 21

Haunter Lvl.21