Odd Fish Meaning

(idiomatic) An unusual thing or eccentric person.

Example: 2001, Paul Theroux, Fresh Air Fiend: Travel Writings (page 355)
  We are constantly told how normal and honorable Thoreau was, yet it seems that we would get much further in understanding him if we began, by conceding that he was an odd fish, full of peculiar conceits.
2010, Stephen Mossman, Marquard von Lindau and the Challenges of Religious Life
  The second work is more promising, but something of an odd fish: the anonymous German treatise Schwester Katrei.