I think I Found Two so far.
#1 - I'm not for sure, but I Just looted an Excellent Sabertooth Tiger Hide off of an Oozing Flesh. - Pretty sure this isn't supposed to be on his loot table.
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#2 - According to the
Key Compendium on the P99 Wiki, Each Level you advance in is supposed to drop a Key that will let you through the previous level mirrors, as well as advance to the next floor. (Other notes states: also grants access to all lower floors) Currently This doesn't work, you get the key but it only works with the corresponding floor it came from.
I just tried Clicking through the First Mirror with a Frosty Key (came from the 3rd floor) on the First Floor and it didn't let me through.