I've never seen anything unusual in the wild - I like some casual birding now and then, but I honestly don't have the patience to wait around for five hours for a rare species to appear, nor do I like to get up at the crack of dawn.
If subspecies are allowed, my best answer would possibly be Motagua Valley beaded lizards. They keep them at Randers Regnskov in Denmark, and these are confiscated animals that cannot be seen elsewhere in Europe. It seems like relatively few Zoochatters have visited Danish zoos, and even fewer have visited Jutlandic zoos (most restrict their visits to Copenhagen). The overall rarest animal in captivity I've seen is probably the Amazon river dolphin, but that doesn't count in this thread as every European Zoochatter seems to have visited Duisburg by now.
If subspecies are allowed, my best answer would possibly be Motagua Valley beaded lizards. They keep them at Randers Regnskov in Denmark, and these are confiscated animals that cannot be seen elsewhere in Europe. It seems like relatively few Zoochatters have visited Danish zoos, and even fewer have visited Jutlandic zoos (most restrict their visits to Copenhagen). The overall rarest animal in captivity I've seen is probably the Amazon river dolphin, but that doesn't count in this thread as every European Zoochatter seems to have visited Duisburg by now.