I don't really consider "The Horn" a DEM, but I know many others do.
Other than Thunder Armor and "The Horn", what's the worst DEM to you (be it a Pokemon, match, etc.)? For me, it's a tie between Trip vs Ash (all), May's baby Squirtle, Tobias's Darkrai vs Torkoal to an extent (Torkoal can take on Registeel but gets beat in one hit by Darkrai? Come on now.), and Skitty vs Dusclops.