See, the players find any Pokemon they want, but their rarity determines how likely it would be for me to say that they catcher them, so, if you found a very rare Pokemon, then it would be quite hard for me to let you catch it, unless your post was very awesome.
That's how it works.
You can just befriend Pokemon instead in any way you like, if it's creative, then you'll be able to keep it, but if it's just like 'here, come with me', I would never let you keep it