No matter how many zoos I've been to that display these particular species, there are just some animals out there that I have absolutely zero luck with photographing. Sometimes it's based on my skill level and available equipment but sometimes I also notice that there are animals that are just plain non-photogenic. Most of which that are high class... Here's a list of animals that I have trouble getting a good photo out of and I would like to read your's if you're a photographer.
Cougars unless they are jumping
African Painted Dogs at any angle
Any gazelle species unless they are rutting
Camels unless they are yawning
Andean condors and all vultures
All snakes
Lizards behind glass
Sloths
Giraffes most of the time
Cheetahs unless they are running
Adult bonobos
All penguins except Blue penguins
Colobus
Guenons
Macaques
Orangutans
Typically Indian Rhinos
All bears, unless they are fighting
Nicobar pigeon - Surprising considering their colors
Saddle-billed Storks - No matter how many times I try, I'm never satisfied
Silverback gorillas
Lionesses
Capybaras
Any small bird
Crocodilians
Bongos
Zebras
Cougars unless they are jumping
African Painted Dogs at any angle
Any gazelle species unless they are rutting
Camels unless they are yawning
Andean condors and all vultures
All snakes
Lizards behind glass
Sloths
Giraffes most of the time
Cheetahs unless they are running
Adult bonobos
All penguins except Blue penguins
Colobus
Guenons
Macaques
Orangutans
Typically Indian Rhinos
All bears, unless they are fighting
Nicobar pigeon - Surprising considering their colors
Saddle-billed Storks - No matter how many times I try, I'm never satisfied
Silverback gorillas
Lionesses
Capybaras
Any small bird
Crocodilians
Bongos
Zebras