Horde Battles are a new feature introduced in Pokémon X & Y where you are able to run into five of a certain wild Pokémon at a time while traveling through areas. These Horde Battles are effectively 1-vs-5 battles, where you use just one of your Pokémon to challenge five wild Pokémon.
Attacks that hit multiple targets are able to hit the opposing horde of Pokémon and are strongly recommended to have in order to deal with these hordes.
Horde Battles do not always include the same Pokémon; related Pokémon may be included in the fight as well, meaning Miltank may appear mixed in with a horde of Tauros.
Here's a list of Pokémon that have been shown engaging in Horde Battles: