Case in point: post #618380
A look at the artist's pixiv page shows that he/she has made many posts tagged with beatmania. This particular one does not have the tag. However, I googled the title just to see if it was original (lol can't read Japanese), and it seems to be a beatmania IIDX song. Title is "お菓子曲" for those who are curious.
Also ran one of the pixiv post's tags (曲擬人化) through Google Translate and got "anthropomorphic songs." So, what I figure is, it's a personification of the song in question. Should it be tagged as such?
Updated by KeliraTelian