Darkrai is a Mythical Pokémon, and it is not available in the main game of HeartGold. You can only obtain Darkrai through special events or distribution methods.
Here are some ways you might have encountered Darkrai in the past:
* Special Events: Nintendo sometimes holds events where players can receive Darkrai in their game.
* Distribution: Darkrai has been distributed through various means like Wi-Fi events or codes.
* Trading: You could potentially trade with someone who has a Darkrai.
However, if you're playing HeartGold right now, you won't find Darkrai in the wild.