Pokémon GO/Pokémon storage

When walking around the map, in the bottom right there's a bar that will show you nearby Pokémon. If you have the Pokémon registered in your Pokédex it'll show up in full colour, otherwse only its outline will be visible. The number of footprints shown below the Pokémon gives an indication of their distance, with three footprints meaning the Pokémon is at some distance. If you're moving in the correct direction, you'll notice the footprints reduce. Once there are no footprints, the Pokémon should appear momentarily.