I think it's on route 7 In Kanto...(at night)
http://bulbapedia.bulbagarden.net/wiki/Houndour
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You catch Houndour at night on route 7 in kanto
You can catch a Houndour in the Altering Cave in Pokemon FireRed.
You get a houndour then evolve it i will update when I find out where to catch houndour
You cannot catch houndour in Pokemon leafgreen either get one from Pokemon emerald or Pokemon colosseum.
Since you can't catch houndour your going to have to trade the Pokemon colosseum houndoom to firered breed it then hatch the egg to get houndour or you can get one from emerald's safari zone.