In some Pokémon Books it is classified as a Legendary Pokémon.
I wouldn't call that thing legendary. You catch it at lv 20? Legendary Pokemon can't breed (Exept Manaphy). Rotom is not a legendary since it can breed.
[EDIT] It should be noted that an unoffical Pokémon handbook, called Pokémon Ultimate Handbook, lists Rotom as a legendary Pokémon. However, this book is riddled with errors (it also lists Delibird as a legendary, for example). An updated version of this book did not list Rotom as a legendary.