Okay I'm not understanding the question much, but ill try to help.
In some areas, finding a horde in tall grass/caves is a 5% chance. The areas are
- Petalburg Woods with Zigzagoon, Wurmple and Shroomish
- Rustruf Tunnel with Whismur
- Granite Cave (First Floor) with Zubat, Geodude and Makuhita
-Granite Cave (Bottom Floor) with Zubat and Aron
- Granite Cave (Bottom Floor 2) with Zubat, Aron, Sableye (if AS) and Mawile (if OR)
-New Mauville with Magnemite and Voltorb
-Fiery Path with Grimer, Koffing, Slugma and Numel
-Jagged Pass with Machop and Spoink
-Meteor Falls with Zubat and Bagon (B1F 2R only)
-Scorched Slab with Zubat
-Safari Zone with Oddish, Psyduck and Doduo
-Mt. Pyre with Shuppet, Vulpix (outside only) and Meditite (outside only)
-Shoal Cave with Zubat and Spheal
-Shoal Cave's Ice Room with Zubat, Snorunt and Spheal
-Seafloor Cavern with Zubat
-Cave of Origin with Zubat
-Sky Pillar with Golbat, Ariados and Swablu
-Victory Road with Zubat, Loudred and Aron
-Route 101 with Zigzagoon and Poochyena
-Route 102 with Zigzagoon, Lotad (AS), Seedot (OR) and Ralts
-Route 103 with Poochyena, Zigzagoon and Wingull
-Route 104 with Zigazagoon, Wingull and Taillow
-Route 110 with Magnemite, Plusle and Minun
-Route 111 with Sandshrew
-Route 112 with Machop and Numel
-Route 113 with Skarmory and Spinda
-Route 114 with Lotad (AS), Seedot (OR) and Swablu (in AS can appear with Lotad)
-Route 115 with Swablu
-Route 116 with Zigzagoon, Nincada and Skitty
-Route 117 with Oddish, Marill, Zigzagoon and Roselia
-Route 118 with Wingull, Electrike and Kekleon
-Route 119 with Oddish
-Route 120 with Oddish, Marill and Kekleon
-Routes 121 and 123 with Wingull, Kekleon and Shuppet
A good way to find them easier is to use the move Sweet Scent, which will trigger a horde automatically if youre in an area that allows them. You can also use honey obtained from a bug catcher in the Pokémon Centre in Fallarbor Town, but you only get one per day. Sweet Scent is better since a variety of Pokémon can learn it, and you can use it infinite times.
A good way to defeat them is to use a move that hits all targets, like Surf, Blizzard or Rock Slide. They are also usually half the level of nearby Pokémon or of a lower evolutionary stage, making them less difficult to defeat.
Source + Experience
