What item do you need to make Latios appear in Pokemon SoulSilver and HeartGold?

You don't need any special item to make Latios appear in Pokemon SoulSilver and HeartGold.

Here's how Latios (or Latias, depending on your game version) appears:

1. Game Version: Latios appears in SoulSilver, while Latias appears in HeartGold.

2. Specific Location: After defeating the Elite Four and becoming the Champion, Latios/Latias will appear at the top of Mt. Coronet.

3. Catch the Legendary: This is your only chance to catch it, so make sure you're prepared!

Important Note: It's possible that Latios/Latias may fly away if you don't engage in battle quickly.