Unfortunately, it is impossible to get Houndour in HG/SS before beating the E4 (excluding trading from another game). After that, the simplest way to get it would be to find it on Route 7 at night. You can also find it in the Safari Zone by placing 4 forest items in the Savannah biome. Additionally, if you have a Pokewalker, you can also find Houndour in the Volcano Path route, which is unlocked by having the National Dex and transferring a total of 10,000 watts to the game.
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