okay, the only possible reason i can think of for thrall having eyebrows is this
orcs shave their eyebrows. it’s generally a masculine thing.
thrall has his eyebrows because he grew up with humans and no one taught him how to shave his face
garrosh shaves his orc eyebrows because he’s too orc to have eyebrows
aggra has eyebrows because why the fuck would you shave something so on point
the mystery is solved
