In SoulSilver and HeartGold, use the Area Customizer to find the Peak Area in the Safari Zone.
Houndoom can be found in the Peak zone.
Peak Area (default). Meadow Area: 4 Peak Blocks
You can catch Lairon in the Safari Zone, but you need to arrange the blocks and he'll most likely be in the Peak area in the Safari Zone.
15 Peak Objects in the Mountain area.
Route 10 and in Safari Zone: Forest Area: 4 Peak Blocks.
Duskull is in the Safari Zone (Swamp) available by placing 28 objects in the Peak area.
you either get it on the peak area on the safari zone or you go into mount silver cave
In Pokémon SoulSilver, you can find Larvitar in the Safari Zone and in Mt. Silver. For the Safari Zone, you can find Level 17 Larvitar in the Mountain area of the Safari Zone however if you want a Level 42 Larvitar then you have to place 5 Peak Objects in the Mountain area of the Safari Zone and it'll appear in there once you have placed the objects in there.
In the safari zone, in the peak area, with 5 plains objects. Hope I helped! :)
safari zone (peak area) if grass object5s are placed may apper in tall grass
in the safari zone area 4
Peak lv 17