It is one of the words that most children associate with wildlife. Does it qualify as a word?  OED shows the etymology of these three letters as:

c1847    Macaulay in Life & Lett. (1878) II. 216   We treated the Clifton Zoo much too contemptuously.

1886    C. E. Pascoe London of To-day (ed. 3) iv. 65   The ‘Zoo’ in time past was as favourite a fashionable resort as Rotten Row.

But the same entry shows that the letters are actually just the first three letters of the word zoological taken as one syllable.  It has doubtless played an important role in awakening passion for conservation, but still: is the thing that the word represents the best we can do to awaken those passions?