I've been messing about with MagicaVoxel after reading some tutorials on this forum and have muddled through a method of crudely constructing voxel models from the original sprites. Basically I take take the front/side/back sprites, scale them all to matching heights, stretch them all to matching width/lengths, and then intersect copies of the blocks.. it gives you a sort of dual prismatic view of the monster. then by copying a side view prism onto itself with a slight offset and subbracting, or intersecting you can use replace to get a model with front back and sides all corresponding to the original sprites. There's some dilemma as to whether to pad out the sides of the creatures, or let the first row of pixels from the front chop into the side, but the corners will need the most correction anyway.
Then you get an ugly picasso looking creature with eyes at the front and side of its head and arm shapes in its belly and extra butts hanging off its arms (in the case of the imp), but the more obvious artifacts can be erased without any particular artistic talent. Using the 3rd dimension and looking at a creature from top down makes some artifacts obvious, and the shading on the original sprites provides cues for where to add depth.
I'm not sure of the etiquette of posting pictures here, I have some screenshots of going through the process I've described above for pulling out a voxel model of a baron (with four legs). This method recovered the 3D horns from the cacodemon fairly well.
For the minute I'll just link to a 3/4 view of an imp that I've tried removign the artifacts from - it looks quite fair.
https://imgur.com/a/qt89TD9
I understand that the reason existing voxel packs don't include monsters is the cost to the cpu.. but I think it would be cool to have voxel monsters, and I'll worry about my own cpu. I'm no artist and I don't think I can voxellate every doom monster animation frame myself, especially not deaths, but a single voxellated monster (besides RDVox's excellent lost soul) as a proof of concept might not be too huge an undertaking.. it would be a start and one by one we could voxellate them all. The voxels look great in doom, 3d monsters IMO never have.