A Booger makes an appearance in "Holly Jolly Secrets Part II," where Shelby points it out as one of the those who got together to watch videos on the floor. In Holly Jolly Secrets Part I, Finn mentioned that the Ice King buried his tapes in a pile of boogers. Another booger appears again in "Marceline's Closet" after Marceline picks her nose. The fact that boogers are recognizable in any way as characters is attributed to that apparently they can have jackets. Finn and Jake avoid the one in "Marceline's Closet," so it is likely that the one in "Holly Jolly Secrets Part II" was simply a joke and that boogers are actually regarded as gross non-characters in Ooo.
The booger is small, round, and light green.