Actually, it's impossible to catch Milotic in any Pokemon Game. But, it's possible to catch its pre-evolution, Feebas. To catch Feebas in Pokemon Emerald, there is a complicated way. But, I know that you can catch Feebas in Route 119. Hope I helped :)
You can't. You have to catch a Feebas & raise it into a Milotic.
To catch a milotic, you'll have to go fishing. There is a chance you'll hook a Feebas. It is a veryrare chance, though. Once you catch it, you have to evolve it. It evolves into Mitolic.
febas evolves into milotic
sex with milotic
You can't catch one in Pokemon Emerald.
Yes, You can catch Groudon 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!
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.
No you cannot catch Celebi in Pokemon Emerald.