The spider-eating guy

[Darkchef]

That's plausible but I highly doubt it for multiple reasons:

Spoiler below!

- The worms seem to be a bit deeper in the mine
- They are a bit bigger than that (though size deviation is arguable, especially among young or such)
- You don't feel any rumbling or hear any worm noises
- The letter in the kennel mentions Wilbur sneaking back out into the mine periodically, tunneling through walls, etc.

Well whatever it was it didn't seem very human to me.