Probably not the most, but Bastiodon comes to mind for something that utterly fails at what it was intended for. OK, maybe it can do SOMETHING...but for a Pokemon hyped by the game as a fantastic wall it's pretty bad.
I summoned a baby to attract a lion. It didn't seem to work, so I tried a delicious baby. Regular baby ATE delicious baby. Horror....horror.~Gameplayingperson