The spider-eating guy

[Sexbad]

There is an illustration of Wilbur together with a dog in the intro of Black Plague when you start a new game. He looks like a wolfman, his mouth seems deformed to look like that of the infected and is wearing some kind of miner's helmet. That kind of mostly solves the mystery.

PLEASE say you're kidding.

That's Red.

There's no answer to whether that's Red or Frisk. The narrator mentions Red when the image appears, but it's also possible that it's Frisk, since he tore up some dogs.