I got into a debate with a friend about whether or not it's possible for an anime to come out before its manga.
He claimed that there were several instances where the anime actually preceded the manga. Is this the case?
P.S.: I know that there are several anime without a manga. I also know that there are manga created from games. That's not what I'm asking. I'm asking whether or not there is a series where the anime came before the manga.
Two more examples of series where the anime came before the manga that I am familiar with are Vandread and Sakura Taisen (a.k.a. Sakura Wars); though to be fair, that latter series was a video game franchise first, before being adapted to anime, and then adapted to manga.
It seems that I may be of little help to you, but there is a manga currently ongoing
and the anime is ahead of both the spin off and the ongoing series. The manga/anime is called
Mushibugyo, and the second series is Joujuu Senjin!! Mushibugyo.
There is no particular rule that anime ought to be adaptions of existing manga. Many anime are original, written by the director and screenwriters. Some of the most popular of these receive manga adaptions after the anime has started airing. Some of these receive both a shounen manga and a shoujo manga, which are illustrated by differing mangaka and either run in different magazines concurrently or run at different times. Some examples of these titles which have both a shounen adaption and a shoujo adaption are:
Neon Genesis Evangelion
Shin Kidousenki Gundam W
Tenkuu no Escaflowne
Anime that later get adapted into manga is not uncommon in shoujo, as shoujo manga magazines an not financially-viable and make less money than shounen magazines, so they are keen to cash in on a title that already has a fanbase (thereby collecting those prior fans who will then begin to buy their magazine and/or graphic novels who otherwise would not have). Some examples include:
Cowboy Bebop and Cowboy Bebop Shooting Star published in ASUKA
Pretty Cure (many varieties) published in Nakayoshi
Ojamajo Doremi published in Nakayoshi
Akihabara Dennou-gumi published in Nakayoshi
Super Doll★Licca-chan published in Nakayoshi
Mahou no Stage Fancy Lala published in Ribon
Aikatsu! published in Ciao and Pucchigumi
Pocket Monster PiPiPi Adventure and Pocket Monster Chamo Chamo Pretty published in Ciao