Edit:
April 2022 Update:
-Previously un-guttable from Zaldon that need to be checked:
Phantom Serpent
-Now guttable and confirmed drops:
Tiger Shark, 285 gil, Duck Ring
-Fish confirmed to remain unguttable:
Lord of Ulbuka
Remora
Ancient Carp
Tusoteuthis Longa
Cameroceras
Dragon's Tabernacle
Quicksilver Blade
King Perch (Malicious Perch equivalent)
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November 2021 new Guttable fish and confirmed drop(s):
Shen, riftdross, riftcinder
Apkallufa, scroll of ice spikes
Yawning Catfish, Seabird ring
Malicious Perch, Patissiere's ring, venom dust
Bloodbotch, Confectioner's Ring, venom dust
Bonefish, multiple Skeleton keys