FOEs are wandering monsters encountered in the Etrian Odyssey series of RPGs for the Nintendo DS and 3DS, they differ from normal enemies in the sense that they are not encountered randomly. In the original Japanese manual, "FOE" is humorously elaborated as meaning "Field-On Enemy"; they appear as glowing orange spheres that, save for the 3DS games, only turn into the monster they represent when the party closes to melee range. The English version elaborates this into the Latin "Formido Oppugnatura Exsequens" (roughly, "ancient and terrible nature warriors"). Continuing the joke, their background music track is entitled "Foedus Obrepit Errabundus" ("the vile, wandering one sneaks up"). It is of note that FOEs are usually on the level of a boss, if not higher, when initially encountered; and that they can enter any battle currently in progress if close enough. Needless to say, it is of the player's best interest to AVOID them, specially if the common monsters are on par with the player's level.
In April 2007, a Flash animation by IOSYS satirized their annoying tendency to pop up at inopportune times during play. It turned them into a popular meme within Japanese imageboard culture. IOSYS depicts the FOE as a literal monster rather than a placeholder: tribble-like orange fuzzballs with the °д° emoticon for a face. This has become the standard way of drawing them, although the nature of black and white doujinshi means white is a common alternate color. It also bears a considerable resemblance to the common fan depiction of kedama, a PCB Stage 4 enemy, which may have been the start of their association with Touhou.
IOSYS's reputation for producing Touhou music and animations has led to FOEs appearing occasionally in Touhou doujinshi and fan art as well. The IOSYS Flash also makes references to "Help me, ERINNNNNN!!", "Marisa Stole the Precious Thing" and Let's Go! Onmyouji.