i guess them being corprus walkers is out, unless the disease managed to alter and live without the heart
but what are they?
new corprus or blight offshoots? the fantasy equivalent of mutants created from red mountain poisons in the air? just golems formed out of ash?

ash creatures are cases of corprus that have been severley mutated by Dagoth Ur, i don't see any reason for them to die after the events of MW.

Or maybe they are corprus walkers that escaped from the corprusarium

My theory is that they're humans who were likely on Solstheim during the eruption of the Red Mountain. The ash covered most of Vvardenfell and parts of Morrowind - likely also reaching the south eastern parts of Solstheim (this could also explain why the Dunmer relocation was so smooth, what with the land being fertile enough to grow gigantic mushrooms in). Anyways, the ash probably hit these humans and some survived - voila, ash creatures.