You can't catch a Haunter in Emerald. You can trade for one, you can find a Duskull/Dusclops at MT Pyre, but you can't straight out find a wild Haunter.
Chat with our AI personalities
No you must trade from FireRed and LeafGreen
The way to evolve Haunter has been, and will always be, by trading. If you have a Haunter in Pokemon Emerald trade it to Sapphire, Ruby, Fire Red or Leaf Green and make sure he isn't holding an EVERSTONE as that will prevent its evolution.
Yes, You can catch Groudon in Pokemon Emerald.
You can't catch one in Pokemon Emerald.
No you cannot catch Entei in Pokemon Emerald.