Slakoth is a Generation III Pokemon, meaning it was introduced in Pokemon Ruby and Sapphire. It wasn't included in Pokemon Emerald.
You can, however, catch a Slakoth in Pokemon Ruby and Sapphire. You can find it in the following locations:
* Route 121: During the day.
* Route 122: During the day.
* Route 127: During the day.
To catch a Slakoth, you'll need to be prepared for its low speed and high HP. Consider using a strategy that involves high-power moves or status ailments.