There is in general only one Pokemon that appears in a swarm for each route. It will just appear in the grass or sand or water on that route, depending on what it is.
The Pokemon that appears in swarms on Route 8 is Croagunk, see here: http://pokemondb.net/location/unova-route-8
So you should just see it walking around in the grass. Try a few different areas and see how you go. However, by now the swarm has probably ended and it will say a new place.