Oni are normally depicted as hideous, gigantic humanoids with sharp claws, unkempt hair and two long horns growing from their heads - they can have varied numbers of eyes or extra fingers and toes and have skin of any color imaginable, thouh red and blue seem to be the most common. Oni tend to wear tiger-skin loincloths and carry iron clubs, which is where the expression "oni with an iron club" came from (which means to be invincible).