#the sort of enclosed and unused open space seen in the picture is only common architecture in a particular time and place

[parsleysagerosemarytimemachine]

'Amorphous vertebrate' is a curious description - how do you end up without definite form, but still have a spine? Which made me think of sea squirts:

Cute little guys; they're invertebrate chordates! At birth, they have a spine and tail, which they lose in adulthood as they attach themselves to some substrate to grow on.

I suspect that this house meat is probably similar - maybe a young mobile form (a fat snake? tsuchinoko real?) that travels in search of a suitable emplacement to mature in. Afterwards, the spine could be reabsorbed, or simply become lost in the expansion of other tissues. I do have my doubts about their cave origins, however; caves with spaces both confined and expansive for a pre-modern house meat to grow within are also unlikely to have much in the way of feeding opportunities. I think it's more likely that it would have developed in either tree hollows or animal burrows, which would similarly provide it both structure and access to prey animals that had previously dwelt there.

Incubomurum domus, commonly known as "house meat", is an amorphous vertebrate that lodges itself in the empty spaces of buildings.

Originally believed to have evolved in caves, it quickly adapted to human dwellings and their ample feeding opportunities. Apart from predating the inhabitants, they may cause structural instabilities to the foundation. Prompt removal is recommended.