I don't think you can unless you hack but I'm not positive that it is impossible to catch it.
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sneasel can't evolve in emerald cause sneasel's evolution doesn't exist in emerald only pearl or platinum or diamond.
You can't unless you trade it from diamond
You can't. You have to transfer it from Leaf Green/Fire Red
sneasel can be found in leafgreen at icefall cave on four island, gligar can be found in the johto area of the safari zone in Pokemon emerald.
No you cannot catch Entei in Pokemon Emerald.