in Pokemon FireRed. can I get Crobat before getting national dex? I wonder about this.
In theory no but I've seen someone do this. I'm not sure though.
Unfortunately, if you're talking about obtaining Crobat through normal evolution in Pokémon FireRed, there is an important limitation: you cannot evolve Golbat into Crobat until you have obtained the National Pokédex. Golbat evolves into Crobat when it levels up with sufficiently high friendship, but FireRed specifically restricts the evolution to after the National Dex has been obtained. This means you can catch Zubat early, evolve it into Golbat, and use Golbat throughout the main adventure, but it won't become Crobat during the pre-National-Dex portion of the game. I actually think this is an interesting example of how the older Pokémon games sometimes had progression restrictions that aren't immediately obvious to newer players. If you already have a Golbat with high friendship and keep leveling it without seeing the evolution, you aren't necessarily doing anything wrong; the National Dex requirement is the obstacle. Once you have completed the necessary progression and received the National Dex, you can then raise Golbat's friendship and level it up to trigger the evolution. So my advice would be not to abandon Golbat simply because Crobat isn't appearing. Golbat can still be a perfectly useful member of the team during the main story, and if you've already invested time in raising its friendship, that effort isn't wasted. Once the National Dex requirement is satisfied, your longtime Golbat can finally make the jump to Crobat.