The first wild Caterpie you face when you enter Viridian Forest is a triggered script - it will always be a level 5 Caterpie. That's technically the Caterpie you're supposed to capture and train, but there's nothing special about it - any Caterpie will do.
Hmm. The most I can figure is you may have the wrong save type configured. What emulator do you use? And maybe a picture of the error. Is it an error from the game? Or is it the emulator? Is the directory savable? The more info you can give, the better.
What you do is you just click on one of these and your status is saved! then if you want to reload your game, go to "Load File" and click the one you registered it on! you can save absolutely ANYWHERE with this system including in the middle of a battle or in the middle of a cutscene!