Houndoom learns Rage as an Egg move,you can get it by breeding a female one with a male Granbull,Snubull,Tauros,Dunsparce,Primeape that know this move
Rage is one of houndooms egg moves : http://pokemondb.net/pokedex/houndoom by breeding a pokemon with this move that is in the same egg groupe as houndoom you get a rageing houndoom