So, this is something that I've come across A LOT since becoming an Undertale fan. Mettaton is not a girl, MTT is a boy. I can prove that he is in fact a dude as well. My older brother bought the Collector's edition of Undertale and with it came a book that basically has bloopers on paper. 

Anyways, I was reading through when I reached Mettaton's page, I was going to flip past it in disgust, (mostly because of how he is portrayed her on WattPad) when I noticed something.

Looks normal, right? Just you wait. 

Toby Fox is the creator of Undertale and if Toby thinks MTT is a boy. Then he's a boy.