¥ does not exist. There is not any monster in NetHack that appears onscreen as a ¥. The character ¥ is not part of ASCII, but requires Unicode or some other character set that supports it, so it cannot even appear in most curses library software, such as the typical tty-Unix game. Now since ¥ is the symbol for Japanese yen, analogous to how $ is the symbol for the dollar, one might guess that ¥, if it existed in the game, would be a currency, as $ is the zorkmid; but twice now in Dudley's dungeon has ¥ appeared as a nonexistant monster.