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:

Missing Pokémon in Emerald
Surskit
Masquerain
Meditite
Medicham
Roselia
Zangoose
Lunatone

And those are all the Hoenn Pokémon Unavailable in Emerald

 
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