You don't say what game this is on, but on most games it is a roaming Pokemon. As long as you do not KO it, it will appear in random routes. You should get given an item to track where it is.
Anyway, to capture it you can do any of these:
- Throw a Quick Ball on the first turn.
- Use a Pokemon with Arena Trap or Shadow Tag to prevent it fleeing.
- Use the move Mean Look to prevent it fleeing (Crobat is good for this due to its high speed).
- Paralyze it or put it to sleep.
- Use the move False Swipe to make sure it only has 1HP left.
- Use a Timer Ball if the current battle has gone for more than 10 turns.
- Use a Dusk Ball at night.