Nice huge dragon dildos. It's like exceptional fetish bingo.
I bet those aren't his, they're his mom's, and he uses them.
Orrrrr they're his and she likes to use them. Either way, they see a lot of action.
So I googled mettaton and read an entire wiki article and now I'm even more confused than I was before.
It was like trying to read about homestuck.
I'll break it down.
Alphys= Lesbian lizard thing
Napstablook= Depressed ghost that makes music
Mettaton= Blooky's former sister that was a part of Alphy's experiments with souls, in which she took the ghost soul and put it in a robot.
Mettaton is a fuckload of mess when you try to discuss it with the fandom. Apparently despite looking feminine or even a dragbot, the fans have found information that Toby himself included, speculating that Mettaton is FtM. But rather than outright say "hey we made a male bot", it became "oh hey they're not a girl robot and they're not a boy robot, this is proof of nonbinary gender!"
Anyone who identifies as Mettaton is usually someone trying to say they're metrosexual but still declare themselves transgender as well. The entire fandom of the game is a nightmare when you get involved.
TL;DR: Alphys took a female ghost, stuck it in a gay af robot, and it's a transgender robot, apparently.(Trans-spirit?)