Some animals have been rare in captivity, and it's usually done as a matter of practicality. Most baleen whales are too large, most deep sea creatures need specific living conditions, and some animals are just rare or live in remote areas and thus are impractical for capture. So, say you're running a world-class zoo/aquarium and you've figured out ways to keep any animal in captivity. What do you get? Me, I'd do some deep-sea exhibits. Only a handful of deep sea animals have been kept in captivity, and not usually in large quantities. Lots of species to choose from, but I'd DEFINITELY want frilled sharks. I think they're absolutely beautiful. For amazement value, a giant or colossal squid. The tank would have a massive window, like you see in aquariums with open sea and whale shark exhibits, so people can see the whole animal and how big it is. I always thought it would be cool to have a whale fall exhibit, where there's a fake whale carcass and some live animals you might see in a whale fall. If deep sea creatures could be kept more easily, I could get sleeper sharks and giant isopods. Finally, I'd have a tunnel tank exhibit, only lit by the bioluminescent deep-sea life. Maybe the start of the exhibit just takes you through a black hallway before you reach the tank. Various lanternfish and plankton, and firefly squid.