Walkthrough
At the end of Route 14 is Route 15, the final path you must make it through in order to reach Fuchsia City. This is the smallest route by far since Lavender Town, so it won't be long until you reach Fuchsia City and the next Pokemon Gym.
Wild Pokemon
Grasses:
Oddish(FireRed)/Bellsprout(LeafGreen) (35%): Levels 22-26
Venonant (30%): Levels 24-26
Pidgey (20%): Levels 25-27
Pidgeotto (5%): Level 29
Gloom(FireRed)/Weepinbell(LeafGreen) (5%): Levels 28-30
Ditto (5%): Level 25
Pokemon Trainers
Picnicker Becky: Level 29 Pikachu, Level 29 Raichu
Crush Kin Ron & Mya: Level 29 Hitmonchan, Level 29 Hitmonlee
Picnicker Celia: Level 33 Clefairy
Biker Ernest: Level 25 Koffing, Level 25 Koffing, Level 25 Weezing, Level 25 Koffing, Level 25 Grimer
Biker Alex: Level 28 Koffing, Level 28 Grimer, Level 28 Weezing
Beauty Grace: Level 29 Pidgeotto, Level 29 Wigglytuff
Beauty Olivia: Level 29 Bulbasaur, Level 29 Ivysaur
Picnicker Kindra: Level 28 Gloom, Level 28 Oddish, Level 28 Oddish
Bird Keeper Chester: Level 28 Dodrio, Level 28 Doduo, Level 28 Doduo
Birdkeeper Edwin: Level 26 Pidgeotto, Level 26 Farfetch'd, Level 26 Doduo, Level 26 Pidgey
Picnicker Yazmin: Level 29 Bellsprout, Level 29 Oddish, Level 29 Tangela
Pokemon Trainers - VS Seeker Rematch
Picnicker Becky: Level 32 Pikachu, Level 32 Raichu
Picnicker Becky: Level 49 Pikachu, Level 49 Raichu
Picnicker Becky: Level 54 Raichu, Level 54 Raichu
Crush Kin Ron & Mya: Level 33 Hitmonchan, Level 33 Hitmonlee
Crush Kin Ron & Mya: Level 51 Hitmonchan, Level 51 Hitmonlee
Crush Kin Ron & Mya: Level 56 Hitmonchan, Level 56 Hitmonlee
Beauty Grace: Level 49 Pidgeot, Level 49 Wigglytuff
Bird Keeper Chester: Level 30 Dodrio, Level 30 Dodrio, Level 30 Doduo
Bird Keeper Chester: Level 48 Dodrio, Level 48 Dodrio, Level 48 Dodrio
Items
EXP Share: From Professor Oak's Aide
TM18: On Ground