So I'm trying to get my favorite Pokemon in my Diamond game. But I cant seem to find it any where. Ive tried to look for it in trading but the name doesn't pop up. Do I need to have it in my pokedex first? If so where can I find one to register it in my pokedex?
You cannot find Houndoom (Houndour) in Diamond, unfortunately.
However, it can be found in HeartGold and SoulSilver (in Route 9) and Platinum (in Valor Lakefront and Route 214 - if you're willing to trade with another game.
Hope I helped. :)