Articuno is the one of the three Legendary Birds.
Articuno debuts in Chapter 25.
Team Rocket is trying to capture Articuno on the Seafoam Islands when Red blunders in, trying to catch Articuno and protecting it from Team Rocket. In the end, Articuno saves Red from Team Rocket. However, in the process, it uses up a lot of energy and soon falls to Team Rocket's persistant forces.
Articuno is forced to fuse with Zapdos and Moltres, creating Zapmolcuno. Red and co. use the combined powers of their respective starter Pokémon to revert their transformation and save the Legendary Birds and the world.