When you arrive at Pal Park for the first time, Professor Oak gives you the PokéRadar and Pokétch feature 20. The PokéRadar allows you to guarantee a Wild Pokémon appearence. However with the National Dex in your possession you will be able to find Pokémon that aren't in the Sinnoh Dex. The way the PokéRadar works is that when you use it in Grass without your bike, you will see the grass start to rustle. Go to it and you will have a Wild Pokémon encounter. However this does not guarantee that you battle the old Pokémon, you can battle any Pokémon in the route.
There is a way to determine it however. The more the bush rustles, the rarer the Pokémon. If the bit of grass also shines, it means a Shiny Pokémon is lurking so be sure to get that

However you do not have to worry if you find an old one, if you battle the Pokémon and accidentally faint it, you will get another chance at it when the PokéRadar goes off again immediately after battle for a good few times. This also helps you to get a specific Gender if for example you are hunting for a Male Kirlia or a Female Snorunt for evolutionary purposes.

Function 20 of the Pokétch also works utilising this. It displays the biggest streak you have had battling Wild Pokémon found with the PokéRadar, just for bragging rights.

This feature is highly useful and will help you fill your National Pokédex as there are a good number of Pokémon available only this way as are their evolutions.

Below are all the PokéRadar-Exclusive Pokémon and the routes/areas they reside in:

PokéRadar-Exclusive Pokémon

Nat No. Pic Name Type 1 Type 2 Location
#029 Nidoran   Route 201
#030 Nidorina   Route 221
Lake Valor
#032 Nidoran   Route 201
#033 Nidorino   Route 221
Lake Valor
#048 Venonat Route 229
#049 Venomoth Route 229
#056 Mankey   Route 225 & 226
#057 Primeape   Route 225 & 226
#079 Slowpoke Route 205 North
Pearl Only
#088 Grimer   Route 212 South
#128 Tauros   Route 209
Route 210 South
#132 Ditto   Route 218
#161 Sentret   Route 202
#175 Togepi   Route 230
#179 Mareep   Valley Windworks
#180 Flaaffy   Route 222
#187 Hoppip Route 205
#188 Skiploom Route 205 North
#191 Sunkern   Route 204 North
#202 Wobbuffet   Lake Verity
Lake Valor
Lake Acuity
#229 Houndoom Routes 214 & 215
Pearl Only
#234 Stantler   Route 207
Pearl Only
#235 Smeargle   Route 212 North
#236 Tyrogue   Route 208
Route 211 West
#241 Miltank   Route 209
Route 210 South
#246 Larvitar Route 207
Diamond Only
#262 Mightyena   Routes 214 & 215
Diamond Only
#277 Swellow Route 213
#280 Ralts   Route 203
Route 204 South
#281 Kirlia   Route 203
Route 204 South
#290 Nincada Eterna Forest
#294 Loudred   Mt. Coronet
#304 Aron Fuego Ironworks
Diamond Only
#324 Torkoal   Route 227
Stark Mountain
#328 Trapinch   Route 228
#329 Vibrava Route 228
#333 Swablu Route 211 East
#343 Baltoy Route 206
#352 Kecleon   Route 210 North
Diamond Only
#355 Duskull   Route 224
#356 Dusclops   Route 224
#361 Snorunt   Routes 216 & 217
Lake Acuity
#371 Bagon   Route 210 North
Pearl Only

 
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