I always thought that exhibit was strictly synonym with exhibition and referred to a multitude rather than single animals/specimens, it is what I always mean when I say exhibit.It seems to me that when I was a child the word 'exhibit' referred to the animals rather than an enclosure, probably because it was the animals that were on exhibition. People would often ask if the zoo had any new exhibits when enquiring about new animals.
just as in a museum the exhibits are the artefacts displayed to the public, and in a courtroom the exhibits are the material objects of evidence displayed to the courtroom, in a zoo the exhibits were the animals.