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.
A † shows that the move was used recently, unless all moves fit this case or there are fewer than five known moves.
Trivia
In the English language version of Pokémon Adventures, it never had an official name. However, Coronis coined the term Zapmolcuno, a portmanteau of the legendary birds' names. In Japanese, it's officially called サ・ファイ・ザーThu-Fi-Zer by Sabrina, a combination of the legendary birds' Japanese names Thunder, Fire, and Freezer.