Swordfish
A swordfish, with the whole fish shaped like a sword, and here called "Gladius maris" or "sea gladiator".
The image was obtained from the source listed below.
Koninklijke Bibliotheek - Medieval Illuminated Manuscripts
Manuscript
Koninklijke Bibliotheek, KB, KA 16 (Der Naturen Bloeme), folio 106v
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Koninklijke Bibliotheek - Medieval Illuminated Manuscripts