Was the song Unravel (OST Tokyo Ghoul) made specifically for the Anime, or was it already made before the Anime, and was just used by the studio for the Anime. If it predates the Anime, was it as famous as it is now, or did it gain popularity after it appeared in the Anime ?
1Most (if not all) anime OP & ED are created specifically for anime. Personally, I never heard of anime reusing an existing song.– Aki Tanaka ♦Apr 18, 2017 at 23:36
I do not know if it was made specifically for the Anime, but the Anime was the debut of the song. The first episode of the Anime having aired on July 4th, 2014 and the single containing the song being released on july 23rd, 2014.
I do remember reading that the Author, Ishida Sui, is a very big fan of Ling Tosite Sigure's lead singer TK, who was asked to sing the opening for the Anime debut.
He personally requested TK to sing the opening song for the anime's first season.
He actively promoted TK's (lead singer) Tour 2016 "Signal to Noise" by illustrating fan articles.
Ishida seems to be a fan of the Japanese rock band 凛として時雨 (Ling tosite sigure) given he has named Tokyo Ghoul:re Chapter 74 after one of their songs.
to support my previous claims as well.
It's pretty common to see Sui promote others via fan drawings and stuff through his Twitter, check it out sometime!