I was originally going to title this post, "Welcome to the Freak Show." I opted not to, in the end, out of a case of sensitivity. The term, attached to circuses and carnivals throughout the centuries, has always carried with it the stigma of degradation. Persons born with disabilities and deformities were collected - sometimes willingly, other times not - for display, for the education and amusement of anyone who could pay. The more striking the deformity, the more popular the attraction.
The same could be said for animals. Just as many rulers throughout the ages kept menageries, many also kept collections of deformed or unusual people. The Spanish conquistadors, for example, found an entire section of Montezuma's royal zoo devoted to this purpose. Perhaps the best known human exhibit was John Merrick, known popularly as "The Elephant Man." Merrick himself mused that his physical appearance may have been due to his mother being frightened by a menagerie elephant while she was pregnant with him.
Of course, if people were willing to pay to see exotic animals, and people were willing to pay to see deformed humans, you can bet that they were willing to pay to see deformed animals. Every carnival or fair that could find one would have a chicken with three legs, or hermaphroditic pigs, or a calf with two faces. If the creature was too maladapted to survive outside its mother's womb, than you could be sure that it would be there in a jar, drifting in formaldehyde. If a genuine "freak" couldn't be found, one could be produced, as evidenced by the countless monkey carcasses that found themselves grafted with fish tails and presented as "Mermaids."
Such animal curiosities seldom appear in modern zoo collections. For the most part, a zoo is not considered a profitable venue for the owners of such oddities to sell their creatures. Most zoos themselves have little interest in such creatures either, preferring to display animals that resemble the natural state, rather than medical abnormalities. The exception to the rule? Color morphs. Zoo and aquarium staff have long had a weakness for abnormally colored animals. Snowflake the albino gorilla was the star of the Barcelona Zoo for years. Black panthers are exceedingly more popular than spotted leopards or jaguars. The white alligators of Audubon Zoo and Audubon Aquarium have become legendary. And, of course, the most famous zoo oddities of all - white tigers.
I'm not sure what the rational behind this is. I suppose it comes down to logistics and aesthetics. Many physically malformed animals have a hard time surviving, which is part of the reason that they are so seldom encountered (besides the fact that they rarely occur at all). Apart from their sensitivity to sunlight in some instances, color morphs pose few challenges compared to their normal colored-counterparts. Also, whereas many visitors find animals with extra (or too few) limbs, eyes, etc to be off-putting or pitiful, white tigers and albino alligators are often seen as beautiful and special.
I will admit, I've also seen a few two-headed reptiles in a few collections. I've never really enjoyed seeing animal oddities (apart, I'll admit, from color morphs), having felt that they're too much focus on sensationalism and drama, too little contribution to the zoos' serious goals of conservation and education. In this I feel they resemble hybrids, though the latter are often deliberately produced.
I suppose it's worth noting, however that for people who aren't familiar with animals, there isn't much of a difference between an exotic animal they've never heard of before, one that is the hybrid of two other species, and one that is exhibiting some sort of abnormality. What they know about the animal is entirely up to the displaying institution's messaging.