Here is two photos of them side by side. LOOK AT THE SIMILIARITIES.
First off, Papyrus' shorts or whatever almost PERFECTLY matches the Knight's.
Second, both have skinny [ ] arms and legs. THE KNIGHT HAS STICKS FOR LEGS TOO, AND WE DON'T EVEN SEE PAPYRUS' FEET, SO HE COULD THEORETICALLY HAVE STUBS TOO!
Thirdly, The Knight has shoulder plates, you know who else has shoulder plates? PAPYRUS!
The Knight is fast af, they could easily get there, open a fountain, and left, in like under a minute. Also, the Knight's "home" is the shelter/Wing Gaster's Wacky Wbunker