Horde Encounters is a feature that first appeared in X and Y, where a player has the chance to run into five wild Pokémon at the same time. In Omega Ruby and Alpha Sapphire, horde encounters are expanded upon and can involve a run in with five trainers too.
These horde of wild Pokémon are usually the same species with a chance of one of them being of a different species. These Pokémon are about half the levels of a single run-in Pokémon and are generally the lower evolutionary stage of them too.