@Cat-Man Blimey, that's a question! Just at San Diego Zoo, and sticking with mammals - off the top of my head and from glancing down my list, so in no particular order - Sun Bear, Spectacled Bear, Pacarana, Giant Eland, Suni, Common Duiker, Yellow-backed Duiker, Red Brocket, Soemmerring's Gazelle, Red-fronted Gazelle, Cuvier's Gazelle, Tiger Quoll, Goodfellow's Tree Kangaroo, Koala, Tasmanian Devil, Fossa, African Civet, Yellow-throated Marten, Tayra, Red River Hog, Bearded Pig, Malayan Tiger, Klipspringer, Snow Sheep, Rocky Mountain Goat, Transcapian Urial, Dall's Sheep, Bharal, Tufted Deer, Bawean Deer, Beisa Oryx, Guenther's Dik-dik, Styan's Red Panda, Southern Hairy-nosed Wombat, Bat-eared Fox, Corsac Fox, Binturong, Striped Hyaena, Aardwolf, Drill, Banteng, Mishmi Takin, Sichuan Takin, Central Chinese Goral, East Caucasian Tur, Japanese Serow, Red-shanked Douc, Baird's Tapir (and probably more I've not thought of - and the full range from things I'd never see again to things now quite common!)
Later in the trip would be species like Pronghorn, Short-finned Pilot Whale, Beluga, Commerson's Dolphin, Florida Manatee, Sea Otter, Banded Linsang, Northern White Rhino, Jackson's Hartebeest, Topi, Thomson's Gazelle, Fringe-eared Oryx, Impala, Slender-horned Gazelle, Bighorn Sheep, Bactrian Wapiti, Timor Deer, Black Wildebeest, Coyote, Vervet Monkey... many, many more.
It was my first serious non-UK zooing and I was only 14 so I shudder to think what I overlooked in terms of birds and herps because I was distracted by the mammals! Definitely my first Gharial, Komodo Dragon and sea turtles though - and my first Harpy, Crowned and Steller's Sea Eagles, and Magellanic and Emperor Penguins. Also Shoebill, Goliath Heron, Dwarf Cassowary...
@Maguari That is an incredible list of species. As you say, some of which are now common and others which are now incredibly rare.
I had no idea you were so young; did your parents take you? 14 was a similar age to my first (and thus far only) San Diego visit, after years of begging my parents (and being young enough to be completely oblivious of money!) to take me. Unfortunately this was in 2013, so the list of firsts was much smaller than yours regrettably. Giant Panda and Puma were the main reasons I went, although I am sure that I am forgetting many less 'glamourous' species.
@Cat-Man Yes, it was a family holiday - we were there three weeks so I was able to monopolise the San Diego days (three zoo days in a row took some convincing - fortunately the Zoo, SeaWorld and WAP were sufficiently different from each other and all so famous that I got away with it!) and then have a couple of quick zoo stops later on as well. Still wish I'd fought harder for LA Zoo though...!
Plenty of non-zoo stuff of course - Golden Gate Bridge, Vegas, Grand Canyon, Yosemite. All good stuff.