The only Pokemon with 0% spawn rates are Ditto and the legendaries

It does, nearby where I live there's a town by this harbor, and people flood the area due to insane spawns of Pokemon and there is about 20pokestops in the vicinity. My friend went down and saw Gyarados spawn on the radar and then he found a big group of people running around looking for it. Nobody was able to catch it, but I believe him, ended up catching a Dragonite there and usually there are 5-6 Magikarp there at once. If the pre-volve Pokemon spawn there, so do the evolved forms, so I've come to learn. Also, if you're the only player in the area, level seems to factor in to what spawns so the higher level you are, the better chances you have a rare Pokemon spawning, same with if you are in a large group. These are all just things I've come to learn on my own as I've been playing.