Red's Gym Badges triggered a fusion machine and created this Pokémon. Team Rocket stole an Eevee in order to research the EeveelutionsVaporeon, Jolteon, and Flareon, which was a mere beta-test for this creature. It nearly destroyed Saffron City. Red, Blue, and Green all did battle with the beast and after a long battle, managed to defeat it. After being defeated, the three legendary birds separated to their original forms and left.
Moves used recently are in bold unless all moves all fit this case.
Trivia
In Pokémon Adventures canon, it never had a proper name. However, Coronis coined the term Zapmolcuno, a portmanteau of the legendary birds' names. Japanese fans also coined the term サ・ファイ・ザーThu Fi Zer, a combination of the legendary birds' Japanese names Thunder, Fire, and Freezer.
Due to it being a Combination of three Pokémon it seems to have the ability to use more than Four moves.