What you do, is declare you're going out into an area, forest, trail, mountain, beach, etc... and that you're searching for an encounter. A storyteller will roll and decide what you encounter, if anything.
When you find one, you then have the option of battling it, if you succeed in defeating it, you can attempt to capture it, sometimes it works sometimes it doesn't. Sometimes they run off before you can really get a grip on them. Rarely can you hope to encounter just the one you want.