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.
you have to trade from another game
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.
In Pokémon Pearl, in order to catch Gengar you can find wild Gastly in Lost Tower and Old Chateau and then after catching one you can raise it until it reaches Level 25 at which point it will evolve into Haunter and then you can trade Haunter to another trainer in order to evolve it into Gengar and then you can catch wild Haunter in Turnback Cave and you can only catch wild Haunter in Old Chateau in the event of Ruby, Sapphire, FireRed, LeafGreen or Emerald inserted into the Gameboy Advanced slot and then you can evolve Haunter into Gengar by trading Haunter to another trainer.
Yes, You can catch Groudon in Pokemon Emerald.
You can't catch one in Pokemon Emerald.
No you cannot catch Entei in Pokemon Emerald.
You can't catch a Bellsprot in Pokemon Emerald. You trade it from Pokemon Leafgreen!
You cant. You have to catch a gastly then envolve it to haunter then trade with someone and trade back haunter will envolve to gengar!
There are many Pokemon you catch that are similar in sapphire and emerald but there are many Pokemon emerald can only get and not sapphire.
Cressilia is a 4th generation Pokemon so you can't catch it in Pokemon Emerald.
You can't catch or obtain other trainers Pokemon in emerald.