Reggie Perrin visualised his mother-in-law as a ____ (12)

I believe the answer is:

hippopotamus

(Other definitions for hippopotamus that I've seen before include "Oppo hams it up (anag)" , "African ungulate" , "African animal" , "Thick-skinned herbivorous animal of tropical Africa" , "Large aquatic beast" .)

I've seen this clue in The Sunday Times and The Times.

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