Animal with a huge mouth (12)

I believe the answer is:

hippopotamus

(Other definitions for hippopotamus that I've seen before include "Large African mammal frequenting rivers" , "Large thick-skinned herbivore" , "Large semi-aquatic mammal" , "Thick-skinned herbivorous animal of tropical Africa" , "Large aquatic beast" .)

I've seen this clue in The Mirror.

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