[@CAS1006]

From what I've seen of Pokemon games they use a similar mechanic to a lot of Japanese games, which is random loot drops. The only difference is that the 'random loot' are the creatures you encounter. The only way to collect them all is to mindlessly repeat the same hunting procedure until the random drop gives you the animal you need. In some cases this could be over in a few iterations, but in some you can be doing the same low-level fights hundreds of time before finally finding the one creature you were after. Only for it to then run away.

I hate games like that, where it's not so much how skillful you are or how well you manage your team, but how lucky you are with the random number generator.