So,I was walking through Route 1 (I think it's Route 1) and I suddenly encountered a Farfetch'd! I had defeated the Champion,which might have caused it to appear,but I'm not sure.This also happened with Poliwag in Victory Road,when I was fishing.
It's clled a Swarm, you'll see it on the billboards. Rare pokemon appear in locations.As for the Poliwag, you were using the Super Rod, which triggered it.
See here for info on swarms: http://pokemondb.net/pokebase/18652/what-does-swarm-mean