Papyton

Papyton is the Slash ship between Papyrus and Mettaton from the Undertale and Deltarune fandoms.

Canon
Mettaton doesn't seem to know anything about Papyrus even if the skeleton is one of his biggest fans.

However, Papyrus watches his shows. Papyrus is an hudge fan of Mettaton and one time, at Hotland, you can call him and he describes Mettaton as his favorite sexy rectangle. He also says that he wishes he could live in Hotland to see Mettaton, even if he said multiple times he was glad he didn't live in this region. He also pretands that Mettaton's ghost is called Happstablook, the happy ghost, even if Undyne doesn't think so. If you go in Papyrus' bedroom, you can find figurines of robots that could be represanting Mettaton. The skeleton also comments on Mettaton's beautiful bishonen eyes. Before Flowey turns into Asriel, in the pacifist ending, Papyrus sees Mettaton's leg and seems to be impressed.

In an ending, if Mettaton becames the king and Papyrus is still alive, he becomes his agent, with Sans. Otherwise, if Mettaton dies, Papyrus never talk about it in a phone talk.

Fanon
People like to pair them because the both have a lot of self-confidence and care a lot about their friends. Some people also say that Papyrus is the only other monster in the Underground with an ego big enough to keep up with Mettaton's.

Most of Papyton comics are representing Mettaton trying to seduce Papyrus, despite him being a really innocent person. In some of them, there is also Sans who, obviously, disagrees with this ship. In some, Mettaton is portrayed as a women and they have children.