Yes, it does count to the bestiary, but Midnight Panther should not become "fiendish" as the occurrence is not common.
"Only creatures which 1) have the occurrence common 2) are not part of a raid, and 3) are not located on Dawnport or Rookgaard, or in boss lairs as well as in arenas can become influenced or fiendish monsters. Also, only lootable creatures can become fiendish monsters."1
I believe it was just one of the bugs, if it really happened. Creatures in fact are usually not spawned as "fiendish", they spawn as regular and might become fiendish. Monster becoming fiendish is a "bonus" same as with the influenced monster, and does not remove it's regular loot, or other properties.
"Fiendish monsters will now turn into regular monsters after one hour; the spell Find Fiend will state how many minutes the creature will still remain fiendish if it were to turn into a regular creature in the next 15 minutes."2