For monsters, I think these files are "M<<filename reference>>INV" and are either .BAMs or .PLTs.IESDP says
The paperdoll image (PLT file) for an animation is named after the animation name (anisnd.ids), a number represending the armor state, then 'inv', hence mber1inv for a bear paperdoll.
ANISND.IDS lists spiders as
0x7A00 MSPI CGAMEANIMATIONTYPE_SPIDER_GIANT
0x7A01 MSPI CGAMEANIMATIONTYPE_SPIDER_HUGE
0x7A02 MSPI CGAMEANIMATIONTYPE_SPIDER_PHASE
0x7A03 MSPI CGAMEANIMATIONTYPE_SPIDER_SWORD
0x7A04 MSPI CGAMEANIMATIONTYPE_SPIDER_WRAITH
These all seem to have the same prefix, but the only -INV.bam/.plt file existing is MSPI3INV.BAM. Therefore, I'm not sure that the prefixes alone will determine the right filenames, but from a quick glance, it looks like the numbers on the left might help in some cases. Since sword is 3 and phase is 2, I'd think copying SPI3INV.BAM to SPI2INV.BAM would give an inventory image for the phase spider animation (if anything would).
I'd be less sure of what to attempt for the minotaur, since it seems it sort of shares a prefix with the mindflayer (which has an inventory graphic MMININV). I'd try MMIN0INV and MMIN1INV to start, then give up.