Most legendary pokemon are set in specific locations that are hard to reach for the player unless they complete certain obstacles to reach that point. There is 4 locations that you can find legendary pokemon: Mt Ember, Cerulean Cave, Power Plant, Seafoam Islands. All these locations require your pokemon have some kind of hidden move either being Strength or Surf or even both to reach these areas and inside some of these places you will need to get past certain obstacles to actually get to the legendary pokemon.
no
And this is in the pokemon leafgreen and firered section, why?
Piplup does not exist in Pokemon LeafGreen or FireRed.
No, you cannot find cyndaquil in LeafGreen.
you should go to gamestop
LeafGreen version does not have Growlithe.
In the grass...
You cannot catch houndour in Pokemon leafgreen either get one from Pokemon emerald or Pokemon colosseum.
there is no such Pokemon
That item doesn't exist in leafgreen.
you can't catch aipom in leafgreen but you can get one from Pokemon colosseum or Pokemon emerald.
There aren't growlithe in LeafGreen. FireRed has growlithe, LeafGreen has vulpix.