For several pokémon I found their evolutions in the wild. For example Tentacruel, Raticate and Pidgeotto. I was wondering if Gyarados maybe is an exception because of the exceptional high evolve cost of Magikarp? I can imagine that they really want you to evolve a Magikarp in this case. So, has anyone found a wild one yet?
Yes. Gyarados is accessible in the wild.
As it does not have a rarity class1 (note the
PokemonClass on Mew or Mewtwo) assigned to it, it may spawn freely (albeit rarely) across the world.
However, due to their spawn chances, it may not be as much work to evolve a Magikarp -- unless you're exceedingly lucky and near one of the biomes in the game that supports Gyarados.
1The data sourced in the above dump came from packet and game reverse-engineering and has since been validated by many people, including myself. It is perhaps, as of now, the best source of information we have on what makes Pokemon tick.
Yes, you can find Gyarados in the wild, although it is very rare. There have been a couple of sightings, here in Australia. It has also been registered in at least two different instances, on a collective spreadsheet for listing Pokemon locations in Sydney1.
#130 Gyarados Darling Harbour, Circular Quay
Gyrados is rarer than Magikarp, but consider that you need 400 Magikarp Candy to evolve a Magikarp. If you find a location accessible to you, that is known to spawn a Gyarados, it would be arguably more efficient than attempting to catch 100 Magikarp.
1 Be aware that the creator, one Bradley Dowling, has been updating this list. However, for reasons that are lost on me, he made it available to anonymous public edit. The list, itself, is often stripped and vandalised. You have to revert to the last version posted by the original author to read through the legitimate list.