Here's my thought on the theory (yes, I tend to extend theories) - but I'm gonna warn, it gets a bit... ...creepy:
What if the variant of peat shown here is actually a fungal infection that can seize control of, disform, and mostly encompass a deceased corpse?
It would explain the moving behaviour to a tee; humans tend to socialize, so it gives a base incentive for the would-be Creeper. If you also look carefully at the creeper, notice how there's black on its feet, rather than brown, or a moderately dark green? Those could very well be parts of some shoes that the peat didn't want to cover.
With peat's explosive nature, the evidence that MatPat presented, and the links I've created, I think we've finally arrived on a proper conclusion of what a creeper is made of, and how they function (as well as why they'd "de-spawn" on occasion).