It is funny that Claybird mentioned it, because I happen to be reading "Dewey" right now, LOL!
It really just depends on the animal, whether the staff is willing to put in the care, and whether the library is equipped with a filtration system to cut down on allergies.
I know my son's Kindergarten class has 2 guinea pigs (and my class in 4th grade had one) - it is part of teaching the children responsibility, nurturing and caring. You can visibly see these kids relax around these animals when they are little bolts of energy all the rest of the day!