in reply to Esper, I've seen 3 wild Pikachu around here, and Jolteon is easy to get because Eevee spawns once every 3 seconds. But Magnemite, Voltorb and Electabuzz just don't exist. Same with Fire, Ice, Rock and Ground types.
I pass about 50 gyms every weekday going to work and back, and I usually scroll through most and there's still some Pokémon I have not seen in them. Most of these are uncommon spawns or non-existent.
Chansey, Alakazam, Sandslash, Dugtrio, Persian, Primeape, Golem, Muk, Onix, Lickitung, Tangela, Seadra, Ditto, Omastar and of course the region exclusives Kangaskhan, Mr. Mime and Farfetch'd.
Pokémon in gyms tend to reveal spawn rates for Pokémon in an area. Unless they just lucked out on an egg or a random fully evolved spawn. Of these I'm most surprised by Alakazam. I'm sure people have it but never use it.