Firered and leafgreen both can catch different Pokemon but there pokedex is the same, here is some examples: magmar only in leafgreen not firered, oddish in firered not leafgreen, ekans in firered not leafgreen, bellsprout in leafgreen not firered, pinsir in leafgreen not firered.
Pokemon Catching Pinsir Pinsir is harder to catch than other pokemon. You need to go to the bug-catching contest after getting the national Pokedex. It will sometimes appear. Sorry for being brief. Hope I Helped! :D