A lot of sites like Serebii mention the percentage chance of finding Pokemon in the wild, but it doesn’t give anything for horde encounter rate or the chances of a specific Pokemon appearing from a horde. Has anyone tested this out or found it in the game files?
Route 120 is an example. I’m talking about without Sweet Scent or Honey.
Not all locations have wild hordes, but where they exist, hordes may be randomly encountered at approximately a 5% rate in tall grass or flowers (but not in long grass in Pokémon X and Y) or in caves.
Bulbapedia also has Horde Encounter Percentages on this page.