Gonna be completely honest, I doubt that any of the species I put on the "Captive" list really fit the criteria of species that few Zoochatters have seen, especially considering that basically all of these institutions are in institutions located in major metropolitan areas. However, I just thought it could be interesting to make a list of species I've seen that aren't held by that many institutions (at least according to zootierliste)
Captive (Mainly just based on the number of institutions on zoootierliste that have them)
Antelope Jackrabbit (Arizona-Sonora Desert Museum)
Apache Trout (Arizona-Sonora Desert Museum)
Australian Whipray (National Aquarium)
Baja California Rattlesnake (Arizona-Sonora Desert Museum)
Barkly Death Adder (National Aquarium)
Black Milksnake (North Carolina Museum of Natural Sciences)
Black Tandan (National Aquarium)
Bluehead Chub (North Carolina Museum of Natural Sciences)
Blue Salmon Catfish (National Aquarium)
Broken-Striped Newt (North Carolina Museum of Natural Sciences)
Brongersma's Pit Viper (Bronx Zoo)
Bullet Ant (Shedd Aquarium)
Butler's Grunter (National Aquarium)
Canyon Tree Frog (Arizona-Sonoran Desert Museum)
Cape Fear Shiner (North Carolina Museum of Natural Sciences)
Carolina Pygmy Rattlesnake (North Carolina Museum of Natural Sciences)
Centralian Rough Knob-Tailed Gecko (National Aquarium)
Cochin Black Turtle (Bronx Zoo)
Colorado Pikeminnow (Arizona-Sonora Desert Museum)
Darwin Snake-Necked Turtle (National Aquarium)
Exuma Island Iguana (Shedd Aquarium)
Flying Steamerduck (Philadelphia Zoo)
Fowler's Toad (National Zoo)
Gila Topminnow (Arizona-Sonora Desert Museum)
Hecq's Shelldweller (National Aquarium)
Humpback Chub (Arizona-Sonora Desert Museum)
Hyrtl's Catfish (National Aquarium)
Indian Roofed Turtle (Bronx Zoo, only holder in the US)
Irwin's Turtle (National Aquarium)
Kihansi Spray Toad (Bronx Zoo)
Kimberly Snake-Necked Turtle (National Aquarium)
Long-Nosed Snake (Arizona-Sonora Desert Museum)
Longtail Salamander (National Zoo)
Milos Viper (Bronx Zoo)
Mojave Desert Sidewinder (Bronx Zoo)
Naked Goby (National Aquarium)
Northern Yellow-Faced Turtle (National Aquarium)
One-Toed Amphiuma (Philadelphia Zoo)
Pine Woods Tree Frog (North Carolina Museum of Natural Sciences)
Piping Plover (Lincoln Park Zoo)
Razorbill (National Aquarium)
Red-Backed Salamander (National Zoo)
Red-Cheeked Salamander (North Carolina Museum of Natural Sciences)
Red Milksnake (North Carolina Museum of Natural Sciences)
Ring-Tailed Vontsira (Bronx Zoo) (definitely unsure about whether or not to count this considering the number of European institutions, but only the Bronx and Nashville have it in the US)
Roundtail Chub (Arizona-Sonora Desert Museum)
Rufous Hummingbird (Arizona-Sonora Desert Museum)
Screaming Piha (National Aquarium)
Shenandoah Salamander (National Zoo)
Silver Cobbler (National Aquarium)
Skilletfish (National Aquarium)
Sonora Chub (Arizona-Sonora Desert Museum)
Sonoran Desert Sidewinder (Arizona-Sonora Desert Museum)
Sonoran Mud Turtle (Arizona-Sonora Desert Museum)
South Pacific Tree Boa (Philadelphia Zoo)
Spotted Blue-Eye (National Aquarium)
Squirrel Tree Frog (North Carolina Museum of Natural Sciences)
Storm's Stork (Bronx Zoo)
Stripe-Necked Musk Turtle (North Carolina Museum of Natural Sciences)
Twin-Spotted Rattlesnake (Arizona-Sonora Desert Museum)
Victoria Short-Necked Turtle (National Aquarium)
White-Throated Snapping Turtle (National Aquarium)
Yaqui Chub (Arizona-Sonora Desert Museum)
Wild
Burrowing Owl
California Condor
Costa's Hummingbird
Devil's Hole Pupfish
Greater Antillean Grackle
Great Lakes Piping Plover
Heermann's Gull
Sonoran Mud Turtle
Sonoyta Pupfish