No one I have discussed this with is aware of a previously created symbol for normal subgroup.
It certainly seems to have the function of a comparison symbol. Our suggestion is to devise a shape in the style of the other triangles listed in Rule XVI (NC page 113-114) using the letter n to identify it as "normal" -- [braille]$t.n]
A tn must describe the shape the symbol represents ("equilateral triangle pointing left").
As a comparison sign, it would require a space on either side.