Here is my list for mammals. A few I'm not sure if they are super rare, but I was incredible pleased to see all of these.
Short-beaked Echidna (Brookfield Zoo)
Brush-tailed Bettong (Lincoln Park Zoo)
Southern Hairy-nosed Wombat (Brookfield Zoo)
Koala (Kansas City Zoo)
Matschie's Tree Kangaroo (Kansas City Zoo, Omaha's Henry Doorly Zoo, Sedgwick County Zoo, Saint Louis Zoo)
Yellow-footed Rock Wallaby (Omaha's Henry Doorly Zoo)
Black-and-Rufous Elephant Shrew (Philadelphia Zoo, Smithsonian National Zoo, Omaha's Henry Doorly Zoo)
Aye-aye (Omaha's Henry Doorly Zoo)
Coquerel's Sifaka (Omaha' Henry Doorly Zoo, Saint Louis Zoo)
Azara's Night Monkey (Omaha' Henry Doorly Zoo)
Red-backed Bearded Saki (Omaha's Henry Doorly Zoo)
Red-bellied Titi (Smithsonian National Zoo)
Blue Monkey (Kansas City Zoo, Omaha's Henry Doorly Zoo)
Beluga (Shedd Aquarium)
Visayan Warty Pig (Saint Louis Zoo)
Chacoan Peccary (Saint Louis Zoo, Sedgwick County Zoo)
Okapi (Omaha's Henry Doorly Zoo, Sedgwick County Zoo, Brookfield Zoo)
Eld's Deer (Sedgwick County Zoo)
Gaur (Omaha's Henry Doorly Zoo)
Gerenuk (Saint Louis Zoo)
Dall's Sheep (Bear Country U.S.A.)
Little Golden-mantled Flying Fox (Omaha's Henry Doorly Zoo)
Greater Spear-nosed Bat (Omaha' Henry Doorly Zoo)
Greater Bulldog Bat (Omaha' Henry Doorly Zoo)
Mountain Tapir (Cheyenne Mountain Zoo)
Baird's Tapir (Omaha's Henry Doorly Zoo, Sedgwick County Zoo)
Red Wolf (Lincoln Park Zoo)
Bush Dog (Saint Louis Zoo)
Bat-eared Fox (Kansas City Zoo, Omaha's Henry Doorly Zoo, Saint Louis Zoo)
Giant Panda (Smithsonian National Zoo)
Ratel (Omaha's Henry Doorly Zoo)
Tayra (Sedgwick County Zoo)
Spotted-necked Otter (Omaha's Henry Doorly Zoo)
Sea Otter (Shedd Aquarium)