Southern Island Latios .

In Pokémon Sapphire & Pokémon Emerald (when player selected Red at the TV at home when first entering Hall of Fame),
with the Eon Ticket (received from event in real life then from Norman in-game ^RS; received from Ruby & Sapphire via Record Mixing ^E),
players get to visit Southern Island and encounter the Legendary Latios.

Latios is the other Euo duo that wasn't roaming in the player's game, and it is holding the Soul Dew.

For players that wants to activate the event without Injecting the Mystery Event (EN/JP)^RS/Gift Record Mixing (save)^RS, read about the event flags here.