Yes it can be caught. not just in trades. yes you can get it in d/p but your can't trade from d/p to emerald (d/p stands for diamond/pearl if you didn't catch that) (also I'm clarifying each responce in numerical order if you didn't pick that one up either) yes you can get it only in leafgreen. . Mantine can be found at the southern most area of Island seven in Leaf Green by Surfing on the ocean water until you encounter one. It shouldn't take long. Any pokeball will do as long as you weaken the Mantine first. Unfortunately I can't remember what levels it ranges from, somewhere between level 30 and 45 I think.
You cannot get mantine in firered only emerald and colosseum can.
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 cannot get mantine in firered only emerald and colosseum can.
Mantine is not available in firered have colosseum or emerald trade you one.
You can't. You have to trade it from quartz or crystal version.
Sorry, you can't get Mantine in emerald
catch a magikarp,goldeen,remoraid
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!
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.