Well as with any Pokémon game, certain Pokémon are missing from it. However since theres like 149 Kanto Pokemon and about 50 Johto that aren't obtainable, I will only list the Pokémon from Hoenn that aren't obtainable in Pokémon Emerald and therefore you must trade:
And those are all the Hoenn Pokémon Unavailable in Emerald
