Mass outbreak

Pokémon outbreaks (Japanese: 大量発生 mass outbreak), also known as swarms, mass outbreaks or wild outbreaks, are a feature introduced in Generation II that allows the player to catch Pokémon that are usually rare or otherwise unobtainable. They appear in every generation after their introduction except Generation VI.

My Pokémon Ranch explaining the general mechanics of a Mass Outbreak

Generation II

Pokémon outbreaks originated in Generation II. In that generation, if the player registered a certain Trainer in the Pokégear, that Trainer would occasionally call to tell the player that he saw or heard about a large number of Pokémon in a certain area; for the next 30 minutes after such a call (in real time, not play time), the swarming Pokémon would appear in the given area with much greater frequency than the usual 1%.

Pokémon Gold, Silver, and Crystal

#	Pokémon	Location	Levels	Notifier	Game
128	Tauros	Route 38	13	Lass Dana (Route 38)	G	S	C
183	Marill	Mt. Mortar	15	Hiker Parry (Route 45)	G	S	C
193	Yanma	Route 35	12-14	Bug Catcher Arnie (Route 35)	G	S	C
206	Dunsparce	Dark Cave	2-4	Hiker Anthony (Route 33)	G	S	C
209	Snubbull	Route 38	13	Schoolboy Chad (Route 38)	G	S	C
211	Qwilfish	Route 32*	10, 20, 30	Fisher Ralph (Route 32)	G	S	C
223	Remoraid	Route 44*	10, 20	Fisher Wilton (Route 44)	G	S	C

Generation III

In Generation III this method allows the player to catch Pokémon that cannot be caught normally and make rare Pokémon temporarily more common. This was possible due to the record mixing feature of Pokémon Ruby and Sapphire. The outbreaks are advised by televisions in houses, where the Pokémon and its location is told.

Pokémon Ruby, Sapphire, and Emerald

#	Pokémon	Location	Levels	Game
270	Lotad	Route 114*	13	R	S	E
273	Seedot	Route 117*	13, 16	R	S	E
274	Nuzleaf	Route 114	15	R	S	E
283	Surskit	Route 102Routes 102, 114 or 117*	15	R	S	E
300	Skitty	Route 116	15	R	S	E

Generation IV

Pokémon outbreaks were largely expanded in Generation IV. Once the player attains the National Pokédex, every day in a particular location, a "swarm of Pokémon" appears, which changes the probability of finding the given Pokémon from 0% to 40%. Outbreaks last only one day and are random events. Information about Pokémon outbreaks can be found in Sandgem Town by talking to Lucas/Dawn's sister or by watching the Sinnoh television ^DPPt, or by listening to Professor Oak's Pokémon Talk ^HGSS.

When players all join the same group, they will all experience the same outbreak on any given day.

The species of an outbreaks seems to be determined using a formula, because it is not chosen randomly when the player starts the game that day. It is also possible that the species is determined the day before, though there is no additional evidence to support this. If the DS clock is changed or a different DS is used, then the game will detect this and will not change the species swarming until 12:00 AM.

Pokémon Diamond, Pearl, and Platinum

By also using the Poké Radar and dual-slot mode in Pokémon Diamond and Pearl, players can get all Pokémon excluding Tangela, Tropius, the starter Pokémon, and legendary Pokémon of previous generations, while Tangela, Tropius and two Legendary Trios can be regularly acquired in Pokémon Platinum.

Many of the Pokémon obtained through Pokémon outbreaks are outside of the Sinnoh Pokédex, and are usually exclusive to other regions. However, the player can obtain information about the current outbreak by talking to, depending on their own gender, Dawn's or Lucas's sister in Sandgem Town, or seeing the news report on a TV.

Due to higher catching rates for Pokémon that appear during outbreaks, they make longer and better chains when using Poké Radar.

#	Pokémon	Location	Levels	Game
016	Pidgey	Route 229	51	D	P	Pt
081	Magnemite	Fuego Ironworks	28-29	D	P	Pt
083	Farfetch'd	Route 221	28	D	P	Pt
084	Doduo	Route 201	2	D	P	Pt
096	Drowzee	Route 215	20-21	D	P	Pt
098	Krabby	Route 226	49-52	D	P	Pt
100	Voltorb	Route 218	28-29	D	P	Pt
104	Cubone	Route 203	4	D	P	Pt
108	Lickitung	Lake Valor	34-36	D	P	Pt
127	Pinsir	Route 229	48-49	D	P	Pt
177	Natu	Route 224	53	D	P	Pt
206	Dunsparce	Route 208	18	D	P	Pt
209	Snubbull	Route 209	16	D	P	Pt
220	Swinub	Route 217	34	D	P	Pt
222	Corsola	Route 230	50	D	P	Pt
225	Delibird	Route 216	32	D	P	Pt
225	Delibird	Route 217	32-33	D	P	Pt
231	Phanpy	Route 207	5, 7	D	P	Pt
238	Smoochum	Lake Acuity	35	D	P	Pt
246	Larvitar	Route 206	14-16	D	P	Pt
263	Zigzagoon	Route 202	3	D	P	Pt
283	Surskit	Lake Verity	2	D	P	Pt
287	Slakoth	Eterna Forest	10-11	D	P	Pt
296	Makuhita	Route 225	48-51	D	P	Pt
299	Nosepass	Route 206	14	D	P	Pt
300	Skitty	Route 222	40	D	P	Pt
300	Skitty	Route 222	36-40	D	P	Pt
309	Electrike	Valley Windworks	7, 9-10	D	P	Pt
325	Spoink	Route 214	22-23	D	P	Pt
327	Spinda	Route 227	55	D	P	Pt
359	Absol	Route 213	20	D	P	Pt
374	Beldum	Route 228	51-54	D	P	Pt

Pokémon HeartGold and SoulSilver

The feature also appears in HeartGold and SoulSilver, where all the wild Pokémon available include all those that were available in Generation II, Ditto, Chansey, and Pokémon from Hoenn and Sinnoh. Information on swarms can now be found by listening to Professor Oak's Pokémon Talk on the Pokégear radio.

#	Pokémon	Location	Levels	Game
113	Chansey	Route 13	23	HG	SS
132	Ditto	Route 47	34	HG	SS
183	Marill	Mt. Mortar	15	HG	SS
193	Yanma	Route 35	12	HG	SS
206	Dunsparce	Dark Cave	2-3	HG	SS
209	Snubbull	Route 38	16	HG	SS
211	Qwilfish	Route 32	40	HG	SS
223	Remoraid	Route 44	20	HG	SS
261	Poochyena	Route 1	2	HG	SS
278	Wingull	Vermilion City*	35	HG	SS
280	Ralts	Route 34	10-11	HG	SS
302	Sableye	Route 9	13-15	HG	SS
303	Mawile	Route 9	13-15	HG	SS
316	Gulpin	Route 3	5	HG	SS
333	Swablu	Route 45	23	HG	SS
340	Whiscash	Violet City	40	HG	SS
343	Baltoy	Route 3	5	HG	SS
366	Clamperl	Route 19	35	HG	SS
369	Relicanth	Route 12*	40	HG	SS
370	Luvdisc	Route 27*	20	HG	SS
401	Kricketot	Viridian Forest	3	HG	SS
427	Buneary	Route 25	8-10	HG	SS

Generation V

The gates in Unova have a monitor inside that will alert the player where an outbreak is taking place.

Unlike previous games, the Pokémon outbreaks can be manipulated by soft resetting the game: turning the console off without saving the game beforehand, but only if the player has not saved their game at all that day. When the game is restarted, there will be a different Pokémon swarming.

Black and White

During winter, the monitor may occasionally not report on outbreaks, which is due to parts of Route 8, where Croagunk can be found, becoming frozen during Winter and not allowing the Pokémon to be caught there. All have the level range of 15-55.

#	Pokémon	Location	Levels	Game
046	Paras	Route 11	15-55	B	W
056	Mankey	Route 15	15-55	B	W
083	Farfetch'd	Route 01Route 1	15-55	B	W
084	Doduo	Route 12	15-55	B	W
102	Exeggcute	Route 18	15-55	B	W
161	Sentret	Route 07Route 7	15-55	B	W
193	Yanma	Route 14	15-55	B	W
204	Pineco	Route 16	15-55	B	W
228	Houndour	Route 09Route 9	15-55	B	W
235	Smeargle	Route 05Route 5	15-55	B	W
236	Tyrogue	Route 10Route 10	15-55	B	W
261	Poochyena	Route 09Route 9	15-55	B	W
285	Shroomish	Route 11	15-55	B	W
311	Plusle	Route 06Route 6	15-55	B	W
312	Minun	Route 06Route 6	15-55	B	W
313	Volbeat	Route 03Route 3	15-55	B	W
314	Illumise	Route 03Route 3	15-55	B	W
353	Shuppet	Route 13	15-55	B	W
360	Wynaut	Route 02Route 2	15-55	B	W
449	Hippopotas	Route 04Route 4	15-55	B	W
453	Croagunk	Route 08Route 8	15-55	B	W

Black 2 and White 2

All have the level range of 40-55 and encounter rate of 40%.

#	Pokémon	Location	Levels	Game
022	Fearow	Route 15	40-55	B2	W2
079	Slowpoke	Abundant Shrine	40-55	B2	W2
083	Farfetch'd	Route 1	40-55	B2	W2
084	Doduo	Route 12	40-55	B2	W2
097	Hypno	Dreamyard	40-55	B2	W2
122	Mr. Mime	Route 20	40-55	B2	W2
162	Furret	Route 7	40-55	B2	W2
166	Ledian	Route 22	40-55	B2	W2
168	Ariados	Route 22	40-55	B2	W2
177	Natu	Route 5	40-55	B2	W2
185	Sudowoodo	Route 20	40-55	B2	W2
187	Hoppip	Route 18	40-55	B2	W2
195	Quagsire	Route 8	40-55	B2	W2
204	Pineco	Route 16	40-55	B2	W2
277	Swellow	Route 13	40-55	B2	W2
284	Masquerain	Route 11	40-55	B2	W2
311	Plusle	Route 6	40-55	B2	W2
312	Minun	Route 6	40-55	B2	W2
313	Volbeat	Route 3	40-55	B2	W2
314	Illumise	Route 3	40-55	B2	W2
317	Swalot	Route 9	40-55	B2	W2
332	Cacturne	Reversal Mountain	40-55	B2	W2
450	Hippowdon	Desert Resort	40-55	B2	W2